They will vary, for example what kind are they? iPhone4/Gadget Giveaways. A catchy phrase... Are they based on a website? Are they USA fans Are they paid fans/likes Have you spammed them? Have you abused the pages with crap? How many fans have blocked you? Generally: 1000 Fans... $5 2000 Fans... $10 5000 Fans... $20 10000 Fans... $40 500000+ Fans... $200+
Catchy Phrases are good for their viral effectiveness, but there's no shared interest or niche... so to market a product can be tricky. The number of people who actually visit the page will be limited- as the majority just like it on profiles because it looks cool and not because they're attracted to your page or product. So those figures which are low ball offers would be competitive. There's 10's of 1000's out there just gathering dust, as the regular user doesn't realize that the page has a monetary value.
It really depends of how you capture fans. you have fans because they like your brand, you force them to become fans or you buy them..
As nvidura said, its important about the fans and how you got them. if they don't respect you and if you can't sell to them then they are worthless but if you got 50 and they can be marketed to effectively then that would be worth alot more