If you're not too concerned with IQ, then the $20 ebay set will do perfectly to see what strength ND you need.
But then once you decide and buy a 'good' brand version, you've definitely spent $20 more than otherwise. (still, it's better than spending $100 on one that is the wrong strength).
I'd say somewhere in the 6 to 10-stop range is a good start for slowing an f/8 landscape down to 1-2 seconds to get waterfall blur. 20 stops might be needed to get any sort of motion blur in an f/2 or wider low-DOF shot.
I actually bought a Canon-brand 72mm ND8 off ebay second-hand for $30 shipped, it's not too bad quality, but for an f/6 or so landscape on a bright sunny beach i'm still getting 1/100s, I need maybe 20-stops to get to 2 seconds to blur waves.
What i'd recommend, if you don't know what strength to get, and do want quality, just get a good-brand Variable-ND (have to ask others which are the better brands, some make weird cross-hatch patterns at certain settings).