Fort an advance booked trip the Hotel Portillo will insist you book an entire week. And it is no bargain: http://www.skiportillo.com/organice-reserve/precios-ski-week/?lang=en
We need to explore what deals we can get.
There is exactly one hotel up there. The good news is that's why it's not crowded, and if you get lucky like Bob Peters did, you ski lots of powder. The bad news is that you will not get any cut rate deals unless it's last minute AND they are not full.
As I've noted several times, South America is a risky ski destination due to extreme snow volatility. My view is if you want to roll the dice, you might as well take your chances with Las Lenas, which has big mountain scale comparable to the Alps. However 80% of that big mountain is dependent upon the Marte chair that operates 40% of the time. There are more hotel options there, so almost certainly cheaper than Portillo.
I'm not saying don't go there; I've been skiing twice in South America. But you need to have a realistic view. You CAN spend $3-4K and have very little in the way of quality skiing if the weather does not cooperate. The odds of this happening are far higher than in most resorts in western North America. This is why all 7 of my Southern Hemisphere trips have had add-on weeks of non-ski activities.
MadPatSki will probably chime in here about his 5 close-to-the-bone South America trips traveling on buses and staying in hostels for modest cost. That can be done but you're still going to be subject to the weather unreliability issues. He's been exposed to some other problems too.
The Hotel Portillo is nice, but it's possibly THE most expensive way to ski South America. In the context of a group that balks at the cost of a Gathering in Aspen or Whistler, it seems an odd suggestion.
Valle Nevado is where you will get the lodging deals. But it gets less snow and terrain quality is "meh" for advanced skiers. I have not been to Chillan, but it probably gets the most snow of the larger places with decent terrain. Chillan also has some lift issues similar though not quite as severe as at Las Lenas.
My feature articles:
Las Lenas 2005: http://www.firsttracksonline.com/2006/07/17/las-lenas-a-ski-resort-guide/
Portillo/Arpa/Valle Nevado group: http://www.firsttracksonline.com/2008/07/24/great-skiing-options-in-chile-lie-close-to-santiago/