Never done one so couldn't comment on price but you can get near 400hp from a 1.8t with less effort than fitting a vr6 motor...
Found this that may be of 'some' help...
Turbocharging a VW VR6 motor
Golf VR6 turbo
Cost will depend on how well you wanna do it and if you are doing the work yourself... you won't get much info on this forum for this sort of conversion as its not an engine that normally resides in an Audi let alone an A3...
If your goal is 400hp and you have a FWD A3 then you will need an LSD at the very least to get the power down else its pretty pointless going much above 300hp which you can do on a 20v with a hybrid...
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