I fail to see why .greg and greg.com differ in value by $184996.01. No one who registers a TLD will escape the market requirement to also hold the .COM. You'll hold the TLD for years and people will still try to find you via the .com. No other TLD has displaced .COM to this point, and I doubt any ever will. So to answer the question, I'd pay the same as a .com just for the sake of owning a TLD: $3.99 per year.
It depends on; 1) Keywords in domain 2) Google PR 3) Number of backlinks 4) Domain age 5) Google status of the domain and lot more... But if you are buying a brand new domain, it usually cost $10-$12 from a reputable domain registrar..