Are foil cover comic books more valuable?

Not necessarily. In fact, many comic books with foil covers have very little value. Foil covers became quite a fad during the 1990s when a speculators craze overtook the comic book industry. Many people began buying special gimmick comic books in quantity, sometimes in unopened cases. The circulation of comic books skyrocketed well past the level of actual reader and collector interest. As a result, many comics from that era have very little resale value. There may be some foil covers that were published in limited editions that could have collectors value. I recommend checking out printed price guides (Overstreet, Wizard, and Comics Buyer's Guide all publish price guides) or online price guides such as www.comicspriceguide.com. Be aware that most price guides list the cover price as the minimum near mint value, but a lot of 1990s comics won't even sell for cover price.