Consult an Overstreet guide. The guide values are generally inflated as most people won't pay anywhere near guide but the OPG has some very detailed information about the Classics Illustrated comics and the myriad of reprints.
Most important thing to know: A ton of reprints were printed with the original indicia which means they have a publishing date from the 40's even though they are reprints from the 60's. Your first indication is a 15 cent cover price. There were no CI comics pubilshed in the 40's with a 15 cent cover.
There is a set of these up on ebay right now describing them as from the 40's but they are not. I messaged the seller about it a couple of days ago but he ignored me so if you intend on buying some CI comics be aware they may not be what they are being described as.