I'm not saying that that's the best solution. A mefi-like $5 cost to join might be better, but then a) PG has to implement the payment stuff, and b) some people will complain about us charging nontrolls to get accounts.
It's possible that there could be some system where if, say, 5 people with at least 200 karma will vouch for you, it's free, but then there's even more complicated code that needs to get written, and people will complain that you can only join HN if you pay or are part of the "in crowd" that know lots of HNers.
I actually think that could work really well though. If for the first month/50 karma, those people were supposed to moderate the sponsorees comments and give them feedback on what they're doing well and what they're doing badly, we might have a lot fewer complaints about all those noobs that are ruining our site.