pshh, it hard to say really.
it depends on the niche its in, the possibility of further income.
for example if i were to buy it could i:
sell shout-outs to others in the same niche?
create a product to do with your niche the followers are likely to buy? (book, software etc.)
create partnerships with websites to share their pictures?
without seing the fanpage i cant really say.
also, (in my opinion anyway) facebook pages are worth less, as its not the page itself that you use to your advantages, its your post. and its hard to deploy certain strategies on a status update or photo share. But that may just be me, as i like to use persuasive media (i see it as kinda black magic xD).
i'd recommend going over to flippa and see if there are any similar pages to yours, and what they sold for / are selling for.