This is a hard one to figure. The value of alters, given that they are just visually modified versions of the original card, are all in the eye of the beholder.
I've collected a fair number of alters, several from a specific artist in Greece (cannot recall his name) as well as from Card Kitty (I'm sure you've heard of them). Personally, I prefer full art/extended art alters that just bring the art out of the rectangle box and help make the cards feel more real and less like a bunch of text on paper.
On the other hand, an altered, or even just a signed card is no longer mint and can be viewed as a damaged card since it's no longer mint.
While some people might go crazy for the cool look you can get from erasing sections of the card printing over the original foil paper, I don't value it very highly, since I don't perceive it to take as much skill as hand painting art onto the cardboard.
To account for the preference of the collector, I say the value is the price of the original with a +/- 15%