RealGM

Currently the Cavs are way over the cap. So if they wanted to sign another big man to say $7m/yr, how much do they have to pay in luxury tax? or are they not even allowed to sign someone to that high at all?

Your question is way too broad. But ...
1 Cant add free agent players to CLE now ...not allowed until July
2 When over the cap, can only sign another player if you have an "exception" that allows it.
3 Tax paid for exceeding the tax limit varies, depending on how far over the line you are and your history of doing so

You might read cbafaq.com where it explains cap and tax rules in great detail.

I will add this also:

Cleveland, using available exceptions, can keep adding to their Tax bill almost infinitely. The only time a team is limited in that respect is if they are hard capped. You can find that information in the FAQ as well.