Quote from BatterysRevengeMy question is, are these cards more valuable than non-signed cards since they're signed by someone who is famous in the Magic world? Or does it depreciate the value?
Very rarely do I see a pro player signed card sell for more than it's non-signed value. The exceptions almost always lie within invitationals (Bob Maher on Dark Confidant, Tiago Chan on Snapcaster Mage, etc.) and are almost exclusively dependent on playability though, since a lot of the older stuff does not command premiums (Sylvan Safekeeper, Avalanche Riders, Rootwater Thief, etc).
Generally no signed card is worth more anyways, but if it is it's because it's signed by Richard Garfield or the artist. In my experience, pro player sigs on random cards are also really hard to move compared to even unplayable commons signed by their artist.
Quote from mondu_the_fatStrictly speaking, a signature counts as a mark on the card. If you get it appraised, it will be graded lower than what it could have been.
This isn't totally true... there are plenty of gem mint (9-10) graded cards out there, even artist altered.
Quote from MagicMousewhen you get artists to sign your cards is it free?
Technically it can be sometimes, but the real answer is it's proper etiquette to always tip. I'll explain a little more how it works below:
There's two ways to get cards signed: The first, is at a convention where you bring your cards to their artist booth; the second, is mailing the cards to their art studio or PO box. In both cases, artists will usually have a limit to the amount of cards you can get signed in one trip (usually 8-12 cards at conventions, could be a lot smaller in the mail). Most are happy with whatever you are fine to tip/donate (and when mailing will ask for return postage) but some have set prices for different quantities. You should always leave something regardless of how few or many you are getting signed.
Artist alters are almost always commission only, but the prices range from only a couple dollars (Anson Maddocks and Mark Tedin were doing $1 alters last I saw them - they use fine tip sharpies only) to $5 each (rk post usually does this - uses a fine tip sharpie and a silver sharpie) to $20 each (Steve Argyle, for instance only uses his paint pens) to more as they get more intricate/time intensive.
No question is a dumb question.