I procrastinated on this, but donated $200 today (the day after the fundraiser ended). I have been reading the site for 4 years, and it has been worth significantly more than $50 per year.
According to the site metrics there are around 15,000 logged-in monthly active users, so if everyone donated $200 that would exactly equal the $3,000,000 target. This made $200 seem like a reasonable number.
As a LW reader, I find it to be a great summary!
Eliezer wrote an article about this phenomenon, where people think they are writing for a less sophisticated audience than they actually are.