You will probably find the prices for constructing a basement shell priced relatively tightly. If you go with a regular building team who then subcontract, then they will take a 10-20% margin on top. If you go for one of the branded basement specialists, then they have to cover the overheads of websites, marketing, project managers and salesmen, and a mark-up for the 'trust' you put in a well-established brand, so the price will probably come in fairly similar.
If you want to reduce the price, you could offer to take on more of the risks of 'unknowns' like the costs of moving drains, a high water table which will necessity pumping during the dig etc, but this can obviously backfire.
The actual costs the builder will incur are pretty straight-forward - a lot of manpower for the digging, concrete, steel, drainage and waterproofing. Plus a whole load of preparation work such as structural plans, party wall agreements, temporary works, parking suspensions, planning requirements such as drainage surveys, impact studies etc. Most of these are fairly easy to forecast. Because of need to do the underpinning in the right sequence, there is a limit how quickly the work can realistically be done.
The discretionary elements are the ceiling heights and the number and type of lightwells, and how much of the basement is under the garden, which is slightly less expensive to dig.
Where the big difference comes in is with the fit-out, as this depends on hugely on what your specify. If you put in a cheap staircase under the existing stairwell, use the existing heating/hot water system, don't move the electrics, use radiators rather than underfloor heating, only have a front lightwell, and keep the drainage requirements in the basement to a minimum, then your will save a lot. But if you want to replicate the sorts of finish in all the marketing materials (expensive flooring, clever lighting, wine rooms, rear glazing, feature staircases, rooflights up to the ground floor etc, moving around heating/electrics), then the costs will ramp up rapidly.
My sense is that you'll struggle to do a decent spec basement on a regular sized terraced house for under £500k, and you can easily spend more if you're not careful. When you come to sell, something with 2.4m ceilings, minimal natural light and a bland fit out is going to be a lot less appealing. It's probably fairly marginal in terms of being able to turn a profit on the project, and that's before factoring in the implicit cost of the 12 months + of disruption to your lives while it's planned and built.
Bear in mind too that in most basement projects that once the basement walls are insulated and built out, you can lose up to 15cm on each side to the width of the room - so you shell will 4.6-4.7m wide on a 5m wide standard terraced house. If your house is much narrower there comes a level at which if will feel very cramped, so is worth bearing in mind. Definitely think carefully about room layout and working staircases intelligently while remembering building regs and fire door requirements.
Going back to the original question, very few builders are out there to deliberately rip their customers off. Their quotes will be what they think they can do the job for and have a reasonable chance of making a 20% margin. Be very wary of those who quote cheap prices - if you go with them, get a quantity surveyor or your architect to really scrutinise what's included and specified (especially in terms of PC Sums), allow a good contingency, and check on companies house that they have a reasonable amount of money in the bank. Get a proper JCT contract for a project of this size, don't rely on theirs. You really don't want your contractor going bust half way through a basement, as no other contractor will want to pick up the project without a substantial premium for the risks they'll be taking on inheriting another builders botched job.
Get some proper insurance too - contractors liability isn't really sufficient to rely on if the worst happens, and standard policies may not cover work below 3 metres (you will be digging lower in a basement excavation.