How much of a tequila connoisseur are you? I've never cared much for silver or gold, and the anejos usually get too much of the dark barrel flavor imparted in them, so I stick with sipping reposados and usually come back with a 750 ml Don Julio reposado. Looking at my Walmart receipt, they were 285 pesos ten days ago.

Or if it's for something special, try the "Premium" Don Julio 1942 which is probably the best I've had. ~800 pesos. At least try to sample one of the premium classes while you're there.

Best prices usually found at larger grocery stores like Super Gigante, or even Walmart if you can find one.