About the same as an unautographed one (Which I sold at Botcon for $15). I've never seen Transfans pay a premium for autographed toys. For most, it's about the experience of getting your toy signed, not the material value.
About the only signed figures I've seen go for any money are signed Optimus Primes. There's a signed 2007 Pepsi Optimus Prime going for twice normal retail, just because it's signed by Peter Cullen.