A Mark McGwire autographed baseball card is worth between $75. -$100. Prices may vary based on condition, and the type of authenticity that accompanies the signature. Signatures that have not been authenticated could sell at half the market value or less.
Collectible items could add value to the price of a signature but not always. As in the case of a baseball card, generally collectors of baseball cards would rather have a baseball card without the signature. The signature would be considered as a flaw in the condition of the card and will lower the value.
A collector of signatures will not always pay extra for the "collectible" baseball card. The autograph collectors main concern is displaying the signature. This is why it is not a good idea to have an expensive baseball card signed. You could devalue the price of the card, and get less for it.
Once a collectible baseball card is signed the value of the card dramatically goes down as a collectible. The card then becomes an Item to display the signature. The bulk of the value relies on the value of the signature of the player that signed the card, and how well the card displays the signature.
In general a signed baseball card is worth about 30% - 40% the price of a signed baseball or the same price of a signed 8 x 10 photo. A high valued card can add some value.