AT&T GoPhone is still expensive but it has a potential and getting close.
Their data rate is now $25/500MB/month (5c/MB), but the data will rollover if you refill it with any of their data package (the cheapest is $5/50MB).
Calling rate is decent at 10c/min, although text is expensive at 20c/txt (they do have a package that will drop the rate significantly down but this shouldn't concern you if you're not using it for text).
Now that everyone is aware about the rates here's why I said this has potential:
The optimal refill PIN is $25 and it's good for 90 days. The good thing is, after you load your account with this $25 you can use the $25 balance to buy the data package above, and then you can just refill with more PINs each month depending on your usage and then converted any amount you want to the data package.
Basically, whatever the amount of dollars you put into the account, you can convert them 100% to data. You can repeat this forever.
For example, if you start with a fresh account:
1. Load up with $25 (your account now has $25 balance and is good for 3 months, or until the balance hit $0). You can do this from
http://myprepaidrefill.com (AT&T website) for $25+tax or buy discounted PINs somewhere else (Target B&M/callingmart.com/etc)
2. Go to
http://paygonline.com (again, AT&T website) to 'buy feature'. If you choose the $20/500MB data, then you will have:
a. 500 MB data (good for a month or until it hits 0 MB. check with *777*3# from your phone)
b. Your account balance is now $5 (that you can use for talk/text as per the rates above)
You can repeat this each month or whenever your dollar balance hit zero. If you don't use it for voice and text at all, you can see that this allows you to basically have a 'data only' line for $25/500MB/month. These are also stackable up to three times so you can have a data line at $75/1.5GB/month.
Like I said, it isn't great if you use a lot of data. But if you have wi-fi at home and at work and mostly need data access for when you're out and about and you're not streaming music and movies like crazy, this is perfect.
Remember that if at any month you're using less than 500 MB or whenever the period ends and you still have data balance, all you need to do is buy another data package. Your data balance will rollover for another month.
Also, I think one of the best things about it is that it's GSM BYOD, so you can use whatever GSM phone you want.
Edit: sorry I messed up the data rate. It's supposed to be $25/500MB/month so my example is all messed up

I'm fixing now.