Get this:
I average about 10 bucks a month on my T-mobile pay-as-you-go plan. No data and 10 cents per minute or text. I don't even have e-mail set up on it, although it is wifi capable! I bought this LG3 new a few years ago for 50 bucks, plus, of course another 50 for screen protector and case (= pure profit). It works great and has a decent screen, camera and other handy apps (Calculator, calendar, clock) my previous old candy-bar-phone Motorola POS lacked.
Nevertheless I am happy with it because I hate phones and I have more than enough e-mail and other internet-based distractions on my PCs at home and work, as well as a laptop.
I've been a long-time T-Mobile prepaid user. They're great for light usage. Their current offering has a $3 monthly minimum that covers your first 30 minutes and/or texts, then 10 cents per minute/text after that.
For data, I got myself one of
these FreedomPop SIMs. $50 prepaid for one year, 1 GB per month. I can't really recommend relying on FreedomPop for calling; my experience with them has been that the reliability is less than great for voice because they do some weird VOIP stuff or something.
I got a dual-SIM phone and this lets me easily use both plans at once, for roughly $8 per month. I use Google Voice for texting. This uses a small amount of data, but avoids the 10 cent charge per text.
That Red Pocket company linked above looks like a pretty nice option as well, but since you said you average 500 MB per month rather than having 500 MB be your maximum usage, their $15 plan might be more up your alley.