It's really not worth that much. It probably depends more on the shoe itself than the autograph. Ben Roethlisberger gives a substantial amount of autographs. That means supply is high, which drives the price down. I'd ballpark the signature as adding $20-$30 to the value.
I have to say that it's pretty lame that you would sell it. Do you think he would have signed it for you if he knew you'd sell it? Certainly not. If an athlete is going to have their autograph sold, they're going to want it to be for charity.
Do you consider yourself a Steelers fan? What are you going to do with the money? I suggest that you do one of these two things:
A) Don't sell it, and never breathe a word to any Steelers fan that you ever considered selling it.
B) Sell it, but donate the money to someone who really needs it.
Otherwise, other Steelers fans will surely excommunicate you if they find out!
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EDIT: Jackie, I wasn't attacking you personally, just giving my opinion.
Do I believe you that you asked this question with no intention of ever selling it? Not really. I hope I'm wrong.
Oh, and about the other autograph sellers on eBay you mentioned... what kind of ridiculous rationale are you using to justify something simply because a lot of other people are doing it? That's absurdly faulty logic and I'm sure you know it!