AviancaTaca is about to join Star Alliance sometime in the next six months. The alliance invited them to join November 10, 2010. The process of formalizing the procedures and joining the alliance, was expected to take 12 to 18 months to complete at the time. Once they have joined, you can earn and redeem LifeMiles on all Star Alliance carriers.
Right now AviancaTaca is running a promotion for buying LifeMiles and getting a 100% bonus. This only applies, however, for LifeMiles accounts that were opened before December 6th, 2011.
You can buy up to 75K LifeMiles in blocks of 1000 miles that cost $30. The maximum number of miles that one can buy (150K) would thus cost $2250 (75 * $30).
The airline tickets tend to be rather pricey within South America. Depending when you are traveling, it might make sense to buy the miles and use them instead of money to secure the tickets.