These days if you're happy to donate 50 BTC to become an official donator then more power to you but you're likely to be in a microscopic minority. Once upon a time that was 50 bucks and less which is when many of the donations would've been made.
*~$100 (BTC didn't got below $2 since theymos started accepting donations in return for Donator and VIP titles).
Sometime i feel that forum is missing a lot of funds by not using google ads. With the sort of traffic, this forum generates, google ads could generate a lot of revenue for the forum, but then it will not remain as clean (ads free) as it is now.
Bitcointalk.org is primarily a discussion forum, not a business ran to maximize income. There are many reasons against having Google ads, such as: privacy, preferring to have more control over which ads are shown on the page, and not wanting to censor more content than necessary just to please Google (to be compliant with their gambling policies).