This outfit makes a big bore TOP END kit intended for the TTR and charges what I consider to be reasonable rates:
I think their rates are reasonable. But the main problem with the TW motor VS. the TTR/XT is the size of case mouth. So you're talking additional costs for splitting cases, enlarging the case mouths, etc. Be forwarned that the only way to get any real bang for the buck is by using the 225 crankshaft. Splitting cases, enlarging case mouths and and swapping crankshafts will generally put you into a whole nuther realm of costs.
Just trying to give you some perspective of costs. This stuff is relatively cheap if done D.I.Y. but can get relly out of hand if someone else does the work.
Ronnydog, Sebastian and others can probably give you a real estimate of costs to build an all-out, reliable TW motor.
Ronny's D.I.Y., Sebastian builds motors for customers.