Well, for 20 minutes of talk time, a 2-dollar card from any carrier would do if the calls are local. If you want International calling, I'd say you should get a 10 or 20 dollar card. As for data, not all cards allow you to access the network.
Telcel, Mexico's most popular carrier, charges 1 peso per minute and 2 pesos per MB on their prepaid cards (they're called "Tarjeta Amigo"). International calling rates are higher, but since you'll need 250MB of data, you'll need a 500-peso card. These cards, fortunately, also have a promotion where they double the amount, so buying a 250-peso card gives you 500. You can mix them according to your needs (a 250 card with a 50 one or stuff like that). Also, these cards are sold in every convenience store out there, so you won't be having trouble finding one.
Movistar is the second largest carrier and their rates are more competitive. Their promotions include free Twitter, Facebook, Whatsapp, and E-Mail navigation with a pre-paid card, but coverage is not that great everywhere. But since you'll be in a 'major' tourist destination, I don't think coverage will be a problem. Cards are not that popular so you might have trouble finding where they sell these, but that's improving.
Iusacell and Nextel are the other carriers here, and while their rates are better than the other two, their coverages aren't so great AND they don't have prepaid card options (they have a prepaid PLAN, but you need to go into their stores to activate them).
Now, in terms of Internet, free WiFi is VERY much available in a lot of public places. Hotels here (most of them anyway) haven't gotten into the fad of charging the Moon and stars for WiFi and still offer it for free with your room. Restaurants, coffee shops (Starbucks!), and some public parks offer sponsored WiFi, and you can get the login password for free or with a purchase.
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