My Yugo M24/47 shoots just as well as any K98 but price wise, the two are not even in the same area code. K98s are a world unto their own and one little stamping here or a mismatched part there has wicked crazy (some would say ridiculous) effect on the price. I'm not brave enough to test those waters yet, though one day I hope to own a true german K98 of some flavor.
Depending on whether you're looking at an M24, M24/47, a M48, M48A, B, BO etc. I don't think $279 is a horrible deal. I've seen nice ones go north of $400 recently, but condition is of course key in determining whether it's a good deal or not. I will say this, they are not really all that scarce, you always have the option to shop around and eventually a nice one will fall into your lap at a good price.
The ammo market for 7.92×57 mm is hit or miss. Sometimes you can find a guy liquidating an estate or just flipping some good quality surplus ammo at great prices. Other times, it seems that corrosive surplus is 85% what you'd pay for new production S&B or Prvi boxer primed/brass cased stuff. Best to stay away from the Iranian/Persian surplus that is being sold for ~$0.30/round as long handgfires and misfires are pretty universally encountered with this stuff. Turkish, Yugo, Greek, or Romanian surplus is all GTG though.
Blicero is the one to ask, I'm sure he'll be along shortly to impart some knowledge.