Mystic is a good place to start, but you need to remember that they are a full service company with substantial overhead costs and a need to make a healthy profit on their sales. What this means is that it costs them money just to have something in their stock and sell it, and then they need to make a good profit to be able to stay in business and for their staff to make a living wage. They buy material by the carton full and pay a small fraction of what they sell things for. An individual seller is unlikely to approach what a large company like Mystic is able to charge. Still, their prices give you a relative value of one FDC or event cover vs. another which is useful to know. Again, spending time on
ebay and ONLY LOOKING at the SOLD auctions will be more helpful in giving you a sense of what you might be able to obtain if you spend your time selling things one or a few at a time. If you decide to sell to a dealer such as Mystic or any of the others you would not be able to get close to what things actually sell for on
ebay since dealers must legitimately cover their operating costs make a strong profit to stay in business.
Collecting these kinds of covers is best done for the pure fun and learning you get from it rather than any expectation of making a profit or even just getting your money back out of it. Inexpensive items like these are a great and affordable hobby. The excitement of finding and buying things on
ebay or at the dealers is definitely worth the very modest cost and much less expensive than most other hobbies. My suggestion is to keep your collection as a memory of your grandfather and build on it for the sheer fun of it.