Unfortunately, "recent" first day covers are not worth much more than double the postage used on them, and, then, only to a collector, unless there is a relevant or important signature on it, or a hand-drawn cachet or drawing is on it, or if the stamp had a low printing (i.e., less than the "normal" number of stamps were printed), or the stamp had a major error (i.e., the image is upside-down compared with the denomination, or it's missing a color), etc. Generally speaking, as with most stamps and antiques, stamps and related products (i.e., blocks, sheets, first day covers, etc) won't achieve any real value until 50 years or so. As such, most true stamp collectors, such as myself, don't collect them with the intent to sell them; we're "collectors", not "speculators" or "investors" (nothing wrong with either of those, but it is very risky). As such, hold onto them, or give them to your kids or stamp collector friends/relatives.
