Prices for posters are not posted on this website for a variety of reasons. First of all, we don't want to have to continually update the prices. The value of a poster will increase and decrease depending on its availibility, the market, etc. The value of a poster will also depend largely on the condition of the poster. So how do you find information on poster prices? Unfortunately, there's no single place that's the best resource. Your best bet is to look at prices from a variety of places then get a feel for what's a fair price. Here are a few places to look.

E-Bay Advanced Search
One good way to see current values for posters is to look at auctions that have just ended on E-Bay. This will tell you what people are paying for the poster these days. Just make sure you check the box that says "Completed Items only".

Poster Price Homepage
This is a compilation of what dealers have sold their posters for over the years. This includes auction prices, store sales, etc. It catalogs the prices over the years as well as the quality of the poster. These prices tend to be a bit high, so adjust accordingly.

I Collect Movie Posters
You have to register (for free) to use this, but I find it a good resource for poster prices. It not only has reasonable values for the posters, but it has the prices according to poster condition. This is extremely handy in seeing how poster grade affects the value. They also sell a book on movie poster prices.

Tomart's Price Guide to Worldwide Star Wars Collectibles Second Edition by Stephen J. Sansweet and T. N. Tumbusch

This book is really out of date, but it's still a great resource for a couple of reasons. The first is that it has a definitive listing of all the Star Wars posters, both American and International, up until Episode I. If the poster exists, this will more than likely have it. It's also good for gauging prices. Though the poster values are very much out of date, it helps give you a sense for what a poster is worth compared to others.