Continental Records Company Ltd. has a great page on assessing the value of your old records.
An excerpt from the site:

How Much are Your Old Records Really Worth

“Many 45s were issued with ‘picture sleeves.’ A 45 RPM picture sleeve is almost always worth more than the record! If you have 45s with original picture sleeves the records (with the sleeves) are usually worth three or more times the normal value of the 45 (in excellent condition) by itself. Almost all rock and roll and rhythm and blues 45s with picture sleeves from the 1950s (even the most popular records and artists) have values that exceed $10.00 per record. Some have values over $100!
EPs (7′ 45s with 4, 5 or 6 songs) were popular items in the 1950s and early 1960s. They were almost always sold with hard cardboard sleeves similar to LPs. If you have EPs and they are missing the original sleeve, then the price will be greatly diminished. If you have rock and roll and rhythm and blues EPs (with the original cardboard picture sleeves) from the 1950s and early 1960s in excellent condition you really do have a goldmine. Some of these items have values that exceed $1000 per record!”

View the website >>