Uh...nothing on that site is legit. Their prices for those stamps are positively outrageous, and they make no mention of certificates for those stamps, which are definitely not the rarities they advertise them as... I would NOT use that site as a guide for what your stamps are worth. In fact, you may as well stay away from that site altogether, unless you want to simply browse their inconceivably high priced items...
Try searching "us stamp 498 used" on
ebay, then click on "sold listings". The sold listings will give you an idea of what its market value is, and no doubt you'll find plenty of 498s on covers which will give you a pretty exact idea of what yours is worth. Hope this helps!