Depends how big and heavy the tender is. The main choice is between twin arm davits and snap davits. Twin arm davits range in price from a few hundered quid (Plastimo), to a few thousand quid (Whitall, Simpson). Snap davits seem to be a couple of hundred quid all in, and theres some nifty new ones where you leave the outboard attached and it keep it the right way up. There's also a sort of tilting frame version that you slide up and on to the transom.