The AK47(7.62X39) is the traditional AK caliber, and many will suggest that is the route to go. One definite benifit is ammo is available pretty much anywhere.
If I could only have one though, I would pick the AK47. The 5.45X39 round has almost non-existant recoil, is generally inherently more accurate than 7.62X39, and if you are a "tactical" type, the ammo and mags are lighter. One downside, 5.45X39 ammo is a gunshow or internet/mail order only proposition, depending on where you live
Fair price? Depends on what you can/want to spend. A SAR1(7.62X39) or SAR2(5.45X39) will start around 350.00(give or take 20.00), all the way up to full customs and pre-bans that will set you back 700.00 on up.