"Best wishes, Jackie Robinson" on the back of a 2"×3.5" introduction card, non graded. What's it worth?

Answer 1

For the significant difference in market between a certified authentic signature and one being offered without a trustworthy COA, it's well worth the authentication fee. After that, the medium the signature is on makes a  material difference. Such a signed piece would be worth decent money bot nowhere near what a signed ball, bat, jersey or even a quality 8 x 10 photo would bring. Robinson died in 1972. You would not find very much in the way of signed bats or jerseys; that didn't go mainstream until the '80s and later.

The PSA/DNA price guide doesn't list cuts or smaller flat items (the 3 x 5 index card was once the standard thing collectors would have signed but the popularity of those has decreased and nobody has those signed anymore).  A signed photo is valued at $1,200. Signed bats or balls are $10k and up.  I would not expect that card to sell for more than a photo, but not authenticated it will be a much much lower price.

Answer 2

A signed cutout piece of paper that was authenticated by PSA/DNA recently sold for $936 on eBay.  I'd be surprised if just a plain card not authenticated would even sell for $100.  It means that much.

Answer 3

Has it at least been properly authenticated? If not get in touch with PSADNA.