So here's a question Magic thread, what kind of value do you all put on foil cards? Do you all think they're worth vastly inflated prices?

I gotta admit, I'm asking this for a reason. I like foils because they look fancier and, on the right cards, look absolutely amazing even if the card sucks. Recently, two players where I tend to play magic opened two foils of two of the new mythic angels. A Gisela and a Bruna. I want them both (Gisela because I want the foil version of her, and Bruna because I want one of each of the mythic angels and Bruna is the last one I need). The guy with the Bruna is looking for an Avacyn (which I have two of) and I offered to trade it to him for the foil Bruna because Bruna's only a 5 (average) dollar card while Avacyn is 10-11 dollar card. Double Bruna's value essentially just because it's foil. Does that seem like a fair trade?

For the Gisela, I offered to trade the guy one of my two Giselas (I have two of those as well) since one of the reasons he so hesitant to trade his is because he doesn't have one, and then make up the difference...until I found out that Gisela (foil) is close to twenty dollars. O.o What? It's the same card with the same effect, just shiny.

Related, I was going to trade an Arid Mesa (and some other stuff) for an Unhinged/Glued Foil mountain which seemed fair (both lands, even if Arid Mesa has a bit more utility)...but why on earth is a mountain, even foil, worth $20 dollars?! It taps for one red mana!