I would probably go for Nicaragua or Ecuador.
First, what kind of Spanish do you want to learn? If you want to learn Spain-Spanish, then of course you need to go to Spain, but if you prefer to learn Latin American Spanish, it's better to stick to Central/South America.
My experience in Chile was that Chilean Spanish was harder to understand than other South American Spanish. I've heard native Spanish speakers make the same observation. It's a beautiful country, though, and for outdoor activities, you can't beat Patagonia. I would probably leave that for a vacation with which to reward myself after learning the language elsewhere, rather than trying to learn Spanish there. If you're into rafting or kayaking, you must see the Rio Futaleufu!
The Spanish spoken in Ecuador is said to be "pure," and I found it easy to understand when I was there. They speak more slowly there, which is very helpful. It's also just a beautiful country full of wonderful people. And... the Galapagos. Incredible.
I haven't been to Nicaragua, but I hear it's also a beautiful country with wonderful people. Not sure about the accent there.
As far as COL goes, I would think Ecuador or Nicaragua would be lower than Spain or Chile, although rural areas in Spain or Chile are probably cheaper than, say, Madrid or Santiago. If you're planning to fly home at all during that year, then of course the cost of flights would be something to consider, and Nicaragua would probably be cheapest.
I've been to western Mexico, which obviously doesn't give me the right to comment on the entire country, but based on what I've seen, I'm just not a huge fan. And unless you go pretty rural, I would imagine that you'd run into a lot of American influence.
