1: Can users setup sub domains in plesk...

2: can i restrict what users use in plesk (exp: 10mb disk space, 1gb bandwidth etc...)

3: Can users setup mysql databases

4: And where can i find an affordable solution to buy plesk?

1-3
Yes (assuming you are the server admin and have enabled permission for your client to do these things - you can also set it so they can't do 1-3).

4
There have been specials posted in the ad forum (Related Offers & Requests), but other than that, I don't have any specific info.