I can't tell specific numbers, but generally signed issues will increase in value at a retailer comic-shop, but even then it depends on what issue, how rare is it, who signed it, even where it is signed, etc. A rule of thumb that I've learned is that most issues are worth as much as the person you're selling it to is willing to pay (from a private collector's standpoint). There are people that use handbooks for values, and start that as the base price, but if your selling it by bidding it could easily go higher than intended.