How much would it cost to fix a piston? It makes a knocking noise in high gear.

What is your question?

Me and my fiancé are buying a car together. The guy said that when you shift in to about 3 or 4 it starts making a knocking noise. I think it's the piston but I'm not %100 sure. I just want to figure out if it's to much to even stress over. Thank you for the help!