I wouldn't pay for it nor contribute towards the game. The reasons, in my opinion, are as follows:
1. I don't really believe in paying that much for non-physical items.
2. These files are not unique. Anyone at anytime (a former dev for example, or someone with the real proto card) could just release the files for free. Essentially, the value of these items are artificial and can be destroyed at any moment, making the person who paid dearly for it a sucker.
3. There are like 700+ other PC-Engine titles to concentrate on. Why worry about one game that may or may not be even worth while or fun?
Personally, I wish the dev or someone would just release the game to the public in order to destroy all these speculators and re-sellers trying to profit from the prototype. If I owned a copy, I'd release it with a note saying "Sorry, you now have a valueless prototype you can't profit from and gouge the community with. GG son".