Stumbled across a pretty good air fare deal on American Airlines' website this evening. You can fly one way from Chicago (O'hare), IL to Puerto Vallarta, Mexico for as little as $75 one-way.
The stipulations for the deal are that you have to stay a minimum of 2 days and a maximum stay of 30 days.
Of course, the price listed above is before international fees. The cheapest I could find was for $75 outbound on a Thursday and $81 inbound on a Saturday. Taxes and fees are another $116.62 for a total of $272.62.
If you want to try to find the cheap rates for this deal, I recommend flying out on a Thursday and flying back on a Saturday.
UPDATE: I found flights out of St. Louis starting at $99 one-way. Friday's seemed to be one of the cheaper days.