Honestly the QJ Megaminxes are going to get you the best quality for your price. You can get them stickered or tiled, but I think they're more common (and possibly cheaper) tiled. A QJ one should always be <$10, and a Mefferts one is $19.90 with free shipping so it isn't THAT much of a difference to go Mefferts unlike with most of their puzzles. You aren't going to find a "bad" megaminx nowadays, but you're going to find different prices.
All your best options are probably here
, since they usually are a bit cheaper and have the advantage of shipping from the US.