IMO, it's the same question you ask about any type of clothing: Do I go for cheap, or quality (i.e., long-lasting)?
If you like variety and 'dressing like a local', you can almost certainly find it cheaper at your location (especially Mexico).
However, if you're going to be there for any length of time, are going to be packing light (i.e., washing frequently), or will be putting the clothes through a workout (excessive sun, surf, sweat), you might want to consider more 'specialized' travel wear, even though it initially costs a bit more money.
[By the way, Ebay, as always, can help cut costs in this arena.]
There's not really a 'right' answer, it's just the way you like to dress and travel. Many people would rather buy ten $1 shirts and throw them away after a few uses than wash one $10 shirt!
Enjoy,
Todd
www.wonderninja.com Buy and Sell YOUR Personalized Services!