How much value does a signed Magic The Gathering card gain?

Signed cards can be worth more or less than an unsigned card. Some collectors see signed cards as marked.

Cards signed by an individual other than the artist are usually worth less than an unsigned cards, unless that are signed by a person of note from Wizards of the Coast, like Richard Garfield.

Signed cards are legal in tournaments per the official DCI Universal Tournament Rules (linked), which state under Rule 28- Taking Notes: Cards used in a tournament may not have writing on their faces other than signatures or artistic modifications.