Oh, boardgames are no trouble at all to playtest. For one thing, as you point out, you can fit several plays in a four hour slot, and if your game plays out in less than ~30 mins you're golden. With RPGs otoh, one usually struggles to fit preamble, game and feedback in that timeframe. Plus, for some reason people (even non-gamers) are much more willing to test board and card games than RPGs.
As regards free vs paid, for various reasons - lack of a regular gaming group, can't afford to go to loads of cons, unkown publisher etc. - I've garnered very few playtesters via the free route, so paid for is about the only option remaining. Actually no, it isn't. Using boosted posts on my FB page to build a dedicated playtest group (simply posting about it in gaming groups on FB in general is pointless) is something I haven't yet tried, so I guess I should explore that avenue before giving up completely.
Btw, since you mention board and card games, I have a card game trundling its way towards publication, prior to which I'm going to upload a PnP version to BGG. If you're interested, check the press release forum there over the next month or so, or if you like I can PM you when it's up.
Anyway, thanks for all your input.