I can't speak for Southern Chile/Argentina cuz I wasn't there, but since it's farther south, I would expected it to offer better/more snow. Santiago area resorts, including Portillo, left me disappointed last August, there were some areas with fresh stashes and nice bowls if you are willing to hike but I would rate their on-piste experience below that of American west coast resorts in CO, Tahoe and PNW. Perhaps I was there during a particularly bad week, but we encountered many areas of ice, exposed rocks or otherwise very solidly packed snow.
Termas de Chillan, about 300 miles southwest of Santiago, was an entirely different experience. The snow was great. The off piste terrain was varied. There are a lot of natural features if you are willing to hike a little bit including natural half pipes, spines, big open bowls that offer fresh line days the snow. We ran into Travis Rice, Scotty Lago, and Jake Blauvelt filming "The Art of Flight" in one of the back country areas. They also rode Valle Nevado days earlier and mentioned the main reason they were in Chillan was that they were disappointed with Valle Navado/El Colorado/Portillo. Perhaps a dumping of snow in the Santiago areas would have altered our impressions. This is a blog of one of the photographers who was covering the crew:
Scott Serfas Photography: Update from Chile. The making of Brain Farm's movie "Flight".
In terms of costs, air from JFK was ~$1100. Visa was $140 (good for the entire validity of your US passport). Minivan was ~$120/day. US license is okay. Lift tix were about $70. Slopeside housing is VERY expensive at all those places, >$200pp/night. They generally charge per person per night as opposed to nightly room rates. On mountain food is also expensive, comparable to US resort prices. For Portillo, Valle Nevado, El Colorado, we actually stayed in Santiago. There were five of us and it was much cheaper to stay at the Ritz in Santiago than to live slopeside. The drive was about 1 hour, half of which was thru a series of hairpin turns - a nice adventure if you are not prone to motion sickness. There are also lots of good restaurants in Santiago for dinner.
For Chillan, slopeside housing is also similarly expensive. However, there is a small town at the base of the mountain access road (about 5 mile drive) that offer lots of cabanas or cabins/log homes. They vary in price, depending on size and condition. We found a place that was nicely sized - sleeping for 10 - and in pretty good shape for about $200/night. Fireplace, satellite TV, WIFI, steam room. There were also hostel type cabanas that offered single beds for $10-20. The folks there had a good international vibe. Aussies, Kiwis, Europeans, Japanese, Americans, etc... As for booking in advance. As crazy as it seems, it's not necessary. There are lots of cabanas. Just drive up to the property and ask them to show you the interior and bargain on price. There was little information on the internet at the time so we didn't book ahead and were a bit worried driving down. But it all worked out just fine. Our Spanish is very limited and we made out okay.
In total, we spent about $2500pp for the 8 day trip, not inclusive of flight. If we decided to "rough" it, I think we could've shaved off $1000. It was a memorable trip, and Chillan was undoubtedly the highlight. If you are more comfortable going with a tour, you might want to try these guys out:
Chile Ski Tour | PowderQuest
I am sure having a guide in the backcountry would add a lot of value, but we just couldn't justify the added expense. If you intend to stay with one of the hostel cabanas, I am certain you'll find people eager to share their wisdom. One thing we didn't do was to hike to an active volcano in Chillan. The locals highly recommended it. I think it was ~2hr hike from the top of the lift (long and VERY slow, no express lifts). I was told the views there were out of this world. And after you are done taking in the views, you can ride down the vast back country. I don't know when I'll be able to return, but I would definitely plan to spend more time in Chillan and perhaps skip the Santiago area altogether.