Is it possible to buy individual Minifigure parts (like minifig heads only) at the LEGO store? (the physical ones, not the online) if so, how much would they go for?
Short Answer: Yes, you could buy minifig heads at a lego store. Caveats: The heads are part of the "build-a-mini" feature of the lego store. So while you could technically just pick 1 head, you would still be charged for the whole set ($9.99 in USA, I believe). So you may as well pick whole minifigs + 1 accessory + hair/hat. Variety of head, torso, legs and accessories is limited and poor...
If you're ok with "generic", then it may not be so bad. If you need to build an army of medieval hun knights stormtrooper with galactic helmets... not so much.
There is the Build-A-Mini station where you can build three minifigs (plus one accessory each) for $9.99.
That said, some stores will let you buy 15 minifig heads and be done with it.
On the other hand, when our local store recently had the ghost pieces, they restricted us to 3 of those per pack of 3 figs.