It's about $5 US for 50 Oxyflux (20mcg), but they're underdosed, so you very well may have to take 200+mcgs of that; but work your way up as Biggie said.
Spiropent is $7 ish, I think, for 20 tabs (20mcgs also).
Last time I was there prices were up in Pesos, and the exchange rate down, not sure how much of a change there's been in the last few months, but it doesn't really matter for the cheap stuff...