El Colorado/Farellones is a great place for sure, Santa Teresa is the fucking tits. Portillo isn't really that expensive though, if you you stay in the dorm rooms it's around $1000 a week depending on what time of year you are there, that sounds a lot, but that includes pass and 4 meals a day. Also the slingshot lifts are the best thing ever invented.
Chillan sounds good too, but I have never got to go there.
Here's some shots I got in Portillo yesterday