Unfortunately, supply and demand dictates the value. Determining if in fact it is a rare collectible comes first. Value is based on the collectors and people who want them. Finding them is the the problem most of the time. See a great article at wikihow.com/Sell-Old-Phonograph-Records. Have you tried a U2 fan site? U2 memorabilia site? (They are the ones who will pay the $$ for your albums.) In your local bookstore they have books on phonograph records and their values. There are a few sites on the web that you can search and find as well.
To see if any have sold on EBay search the SOLD sales. Sorry to do this here but many people aren't aware of this tool. Go to EBay. It will ask you your keywords and category and next to that it will say Search..and next to that in small blue type it says ADANCED SEARCH. Just click on ADVANCED SEARCH. Then put in your keyword/s (U2 in your case) and go down to the section "Search Including" and check the box for COMPLETED. Then click the blue Search button. This will pull up past transactions to see if they sold and what they sold for.)
Not saying EBay is your only option, it isn't. But sometimes it can help just knowing what people are paying for like items.
