Your better deals will be on ebay, but you need to get lucky too. From a dealer, a signed First Ed. of one of his standard-release books in fine condition will run you anywhere from about $200 to over $2,000. You should look around at this site if you are considering buying a signed book. There is a repository of signed manuscripts that mjp has put together which will allow you to compare authentic signatures from 1969- to the book you are considering.
Even then, there are some good (and plenty of bad) fakes out there. There is a forum here for posting signed books and folks will give you their yea or nay. Very helpful, but if it's an ebay listing, you've just alerted us all to a signed Buk we may also want. ;)
abebooks.com is a great resource for figuring out what is out there, but there are some pricey volumes. I will give you one bit of advice regarding dealers: Jeff Maser bought out a large amount of Black Sparrow a few years back, and he is totally legit. I have three signed books from him, and they are all mint and completely genuine. Google "Jeff Maser Bookseller."
Another option is signed New Year's greetings from Black Sparrow. They are very small, usually printing only one to four poems, but some were signed by Bukowski, and they tend to be a bit cheaper. I have a signed version of "Luck" from 1987, and it's about 4" sqaure, and the signature page includes the drawing of a little man with his flask. You can probably get that for under $200.
