But perhaps signing a 24-month deal on a 6-month old device isn't the smartest of moves
A "special offer from AT&T" has just showed up on the Play Store listing for the HSPA+ Nexus 7 that shows a $100 credit offer for signing a service contract with your new tablet purchase. Similarly to the deal offered by T-Mobile on the Nexus 4 early-on, Google seems to be making small carrier partner deals to try and get some people locked into contracts with their devices even from the Play Store. This deal stipulates that if you sign a 2-year contract of at least $14.99/month (which gets you 250MB of data,) you'll receive a $100 service credit to put towards that data for a few months.
This may seem like a great deal up-front -- as is the case in most contract situations -- but when you think about it this really isn't. You're signing up for 2 years of service to save $100 on a device that's already 6 months old, which in the long run means very little considering the cost of data for these plans. Most users will likely want at least 3GB of data, which will run you $30 a month (before taxes/fees.) Taking the true cost of ownership into mind, getting basically 3 months of free service in return for being forced to pay for another 21 months seems a bit less of a great deal -- especially when considering you're still having to pay the full $299 for the tablet.
If you're an existing AT&T customer, you're far better off adding the device to your current MobileShare plan. If you're interested in having just the tablet separate, you're going to be better served by AT&T's own prepaid options that let you use data one month at a time and have the same monthly rates. Remember, the HSPA+ Nexus 7 also supports T-Mobile's frequencies, which opens up even more choices as well.