It is best to have a Baseball card signed that has a low value, and has less of a chance of growing in value in the future. Once a baseball card is signed it loses it's value as a collectible card, and becomes an item to display the signature. It is less appealing to the card collector, and the collectible becomes the autograph. The bulk of the value will rely on the autograph, not the card.
In general, a collector 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. An inexpensive card will benefit but, if the value grows in the future it will be held back by the growth of the value of the signature.
In the world of collecting Autographs, signed baseball cards have a lower value than signed photo's, and baseballs. There is a small community of collectors that do collect signed baseball cards, and this keeps the value down. They are in less demand.
It would cost you the same amount of money to have a Baseball Card signed as it would to have a Photo signed but, the photo will have a higher resale value. The baseball will cost a little more to have signed but, will have a greater growth in value.