Thanks for the info and guidelines on your requirements. Certainly a steel sheet monocoque is quicker to put together than either a spaceframe or a rivetted aluminium tub, so if you are looking at a limited production run in the future it makes more sense this way, and the tubular rear subframe section gives you a lot of flexibility in modifying just that rear tube section for different drivetrains. There MAY be a possibility of even making the rear tubular frame a bolt-on item to make life even easier, it depends how the design develops.
I would recommend you run at least a four point roll bar for occupant protection - the roll cages for Lotus backbone chassis turn 'inwards' at their lower ends and bolt onto the side of the backbone section. I would envisage having a roll cage sized tube (around 1.75" OD) welded to the rear of the sheet steel centre monocoque, running across the full width of the car, to which the main roll hoop can be welded. This can then be nicely triangulated into the monocoque, and also tied into the rear subframe, giving a nice strong structure. Potentially you could run another tube across further forward, by the occupants' feet, linked to the rear tube by two bars running along the sills, to which the front legs of a full roll cage could be attached... it depends how far you want to go down the roll-over structure route.
I have had a look back over the original estimate I gave, the rear subframe will actually be a similar structure to what I would have done for an ali tub so the cost will be similar, around £300-400 for a subframe/roll bar design. Suspension design, again, similar in cost, around £1000 for complete suspension and steering design with relevant CAD files for pickup points.
For the sheet steel tub including front suspension and steering mounting points, the cost will be a bit cheaper than the aluminium tub as the jointing is more straightforward in steel, plus the ability of steel to absorb localised loads without tearing is superior which makes our lives easier. I quoted you around £2000 for a piece-by-piece design in ali, I would think £1200-1600 is more likely in steel, this would include CAD files of the individual sheet steel elements of the tub.
The reason I asked you earlier whether it was just the design you wanted or a design and build, is that with the sheet steel element of the chassis I would probably create some 'basic' CAD files of the main parts once I have the design laid out, and get those cut by my laser cutters. Then as the tub goes together, any additional holes, cut-outs etc. can be put manually into the cut and folded pieces, plus also added to my CAD file so that any future tub construction will have all the relevant holes and shapes in it. This ongoing sort of process is fine for a design and build, but if it's just a design then I have to make sure that nothing is being missed out so the design process can take a bit longer.
So, to fabricate and TIG weld the rear tubular subframe structure with (say) a four point roll bar in seamless steel tube is likely to cost around £1400-1800. For a jig for this structure, it depends on how intricate you want the jig to be - if it is just a guide for say the main frame tubes and main roll hoop position, it will be cheaper than a complete, multi-piece jig with facilities to clamp every piece of tube onto it... let me know what your thoughts are.
For the suspension wishbones, we always do jigs for those anyway, from past experience I would allow, for fabrication and TIG welding of wishbones and jigs (8 off) around £2500, plus whatever ball joints and bushes/rod ends you want to use.
For the steering, there are a lot of options on racks. We usually use Mk 1/2 Escort for a manual rack as it is readily available in several different ratios at reasonable prices (around £120 if I recall). You need to add outer tie rod ball joints or rod ends to this unit. Depending on exact design criteria, we sometimes need to narrow the centre of the rack to get all the joints to line up correctly, we charge £140 to narrow the inner rack bar and outer rack body if required - though I suspect with the likely width of your car, this won't be necessary. Allow £200 to mount the rack to the tub.
Engine and gearbox mounting to the rear subframe, labour charge would be around £450 plus whatever motor/gearbox rubbers you want to use.
Rear suspension mounting to the rear subframe, again from past projects I would envisage around £500-600 to get this all on, another £200-250 for a suitable jig which would clamp to the subframe and give you your mounting bracket centres for a future build. Regarding dampers, there are obviously a myriad of options here, we probably need to discuss those - but the mountings are all pretty much the same and the cost of mounting rear dampers is included in the price above. Allow the same for front suspension and damper mountings.
That basically leaves the tub. The cost for fabricating the tub is a tricky one to estimate, as at this stage I have no good idea of how complex or simple the structure will be internally, to absorb and distribute the loads. As mentioned before, the specifics of taking the suspension point loads in a safe and sturdy manner is the most critical aspect of a sheet structure, and is where the time will need to be spent. As an extremely rough guide, a simple 'first build' tub might take 40 hours to assemble and weld, a complex one twice that long. At £45 per hour that equates to £1800-3600 in labour, then the cost of the laser-cut pieces - though that shouldn't be too high, maybe £600-ish. Future tubs will be cheaper as all the holes and shapes will be there in the laser cut pieces, plus all the relevant jigs can be used to position everything, reducing labour time considerably. I am including the cost of some simple jigs in the tub price, you won't need much here as the flat or folded sheet pieces will tend to only fit in one position and will up to a point 'lock together' in one way only.
Regarding uprights, I have had success in the past using TVR Cerbera units - these are aluminium units with bolt-on steering arms, ball joint mounting plates and hub/bearing assemblies which give you a lot of flexibility in how you package everything. They are also available brand new at reasonably prices - the upright is under £100 + VAT, the complete hub/bearing assembly around £75 + VAT, for example. All the hubs come with internal drive splines (the TVR rear hub output flange is under £100 plus VAT), for the front you use a nut and bolt with machined washers to secure the hub into the bearing assembly. There are some nice brake kits using AP calipers for these upright/hub assemblies readily available.
I think that covers it for the moment, it's just damper choice is rear subframe jig that needs some discussion on to get an estimated price.