The first thing to say is that this is not at all a long-term solution since the precision that a trackpad can get It is not comparable to that of a professional digitizing tablet, but for a fix it is worth it.
InkLet bases its operation on the sum of trackpad + stylus, and the truth is that if it works like in the video the results are really surprising and worthy of mention, since the software is honestly very successful.
Regarding the price, the application sells for $ 25 while the stylus must be purchased separately.
Source | Apple weblog