Tmobile offers free data, although at edge speeds, and unlimited texting in mexico. Calls are .20 a minute. This is with the simple choice plan which has from 1gb of data to unlimited. It is technically not prepaid but is no contract so you can cancel at any time, like you would on a prepaid line. Honestly though depending on how long you would vacation, if you plan to stay with sprint it may be more cost effective to get a cheap phone and use a local mexico prepaid carrier. If you decide to switch to tmobil however you will always have a phone you can use while you travel if you get to mexico often or even annually, without having to switch phones or pay for prepaid minutes or an unlocked phone every time you travel. You could use wifi calling instead of paying .20 per minute for talk as well.
Edit: I just went on sprint's website and they have what they call international value roaming. Unless you particularly want the features of the 640 or the wifi calling, sprint offers global roaming data and texting in mexico already.