1999 Pontiac grand am 4 cyl. look like major problems!?!?

[Michila Noell]

my 99 grand am is going through water like crazy, no leak tho, its like the water just disappears. when the water gets low, it over heats, we've never seen any white smokey look come out of the exhaust. i've had a few people from friends to relatives (who know ALOT about vehicles) look at it, cant find whats wrong but is pretty confident there is no water/coolant in the oil or vise versa, they suspect i need a head gasket replaced, is that what it sounds like? and to have a head gasket replaced, what am i looking into price wise?
Also, if it makes any different, we can put water in it once a week.. we put it in every other day, usually not much unless its really hot outside, but we add water just so the water pump doesnt get low while we are driving it.. on hot days when my fiance drives it to work, he'll go out on his lunch break and put water in it..

 

[1999grandam2.4l]

If you've been putting water in it for a while then there won't be antifreeze to make a thick white smoke, just a thin steam. Soo, possibly the head gasket yes. Has the water pump ever been replaced?