Well i have bit my tongue long enough and listened to the usuall 10k trotted out by people who still have a TT , i dont think anyone on this thread has mentioned the cost really depends on what kind of power you want, 500hp is very easily achieved at way way under the figures bandied about however 600hp + is going to be up in the 10k region.
There are items you really need to spend good money on such as the turbo (around 1k for a 500-550hp) but there are others where you can save, is a £1500 tubular manifold with the potential to make 800+ hp really necesary, the answer is no if you dont intend going over 600hp and it really makes me laugh when people spend this kind of money then only run 550hp when a £300 cast manifold would have given them a torque curve to die for (yes i know "future proofing" ).
Do you need the latest whizz bang ECU with a total cost of around 2k to run 500hp, the answer again is no, at this power level your still using most of the silky smooth stock ecu and a piggy back is more than adequate.
Granted you will need to ensure all your ancillaries are in good nic (including rad, brakes, suspension, tyres).
The other big saving could be labour, if you know which end of a spanner is which fitting a single turbo system really isn't rocket science , and there is more than enough info on this forum to help out the "garden shed" mechanic.
So my point is it all depends what power you want and believe me 500- 550hp is pretty damm quick and quicker than 99% of cars on the road.