People who are into autographs, would be willing to pay more, but auctions probably don't go high because it isn't a huge market (and so many toys have been signed over the last dozen years of various conventions, the concept has probably flooded the market by now).

If I were into Autographs, I would also make sure it has some sort of proof, as anyone could do a scribble on an Optimus Prime box and claim it is a Peter Cullen autograph. Most are fairly messy with their marker-pen signings, and wouldn't be too tough to replicate with another marker pen.

For someone who doesn't like autographs, I wouldn't even buy something signed, as it does deface the packaging/toy for me.