We have several posts on this forum about the value of FDC's. Please use the "search" function found on every pge. You can also check places like eBay an Delcampe, plus the American First Day Cover Society at www.afdcs.org .
FDCs are a classic case of what was once a huge fad that is no longer so popular, so their value has plunged. Of course there are exceptions, but 90% of the stuff is worth almost nothing. They are a good source of fine used sets for people that can be bothered to soak them off, and who don't mind the FD cancels.
However, most non-stamp-people still think FDCs are as popular as they ever were. Wrong.