A grinder like that costs between $100 and $250 new, depending on who makes it.
While it's not the best for making big knives because of the size, it's pretty good as a starter and for doing small work. You can strengthen up the platen if need be. Apretty good selection of belt grits is available, too.
If everything works and the wheels aren't chewed up, $50 is a bargain, IMHO.