Not sure what it's worth either except not $20 or anywhere close. I love Steve Ritchie in a bromance kind of way, but his signature isn't really worth anything monetarily. It's cool no doubt, and it's awesome that he takes the time to sign stuff for people at the pinball shows, but it's really just to remember him by as a memento. It's not an eBay collectible. Picasso can't pay for dinner by signing the table napkin, and you can't charge $20 for a worthless piece of plastic because Steve Ritchie signed it.

Anyone can get his signature at the signature portion of Pinball Expo, so I'd say his signature really doesn't add much value.