Head Gasket??

Hi guys,
I recently purchased a 2000 Wrangler Sport w/80k. THREE days after the warrenty expired, I started to have problems. Temp Guage would shoot up so checked coolant and it was almost 1 gallon low. Added coolant and was fine for about a week (maybe 300 miles). Started happening again. Checked coolant again and had to add another gallon.. No antifreeze leaks anywhere and can't smell it near exhaust. Check oil and it did not appear to be milky, looked fine. Think it's the head gasket?? How can I tell, best way to check.
Also, if it is, how much is an appx. rate for replacement at mechanics??
Thanks in advance for any help

I would first look around the water pump for any indication of leakage from the bottom weep-hole. To check for headgasket issue, you can pull your plugs out, taking note which one came from which port and see if you find one that has almost no (clean) or white deposits on the plug. This indicating,anti-freeze getting into that cylinder and you have a H-gasket issue. The best way to find out what you have, if there is nothing obvious, ie. leaks at W/P, or rad. tanks. Have the system pressure checked at a reputable garage. This will usually pin-point the problem more accurately.