@Speaker - you have a couple of contradicting requirements...
- "No contract" in most cases means you will pay full price for phone (or pay it in instalments, which is same). Your budget of $100/phone will not work here even for basic phone.
- Add-on memory excludes iPhone. From Android/Windows phones, not too many phones support extra memory.
- 500mb/mo for three people - you will eat this in a week, or two.
You can get really cheap phones if you go prepaid. You can get really decent phones for under $100. You will get to have separate data plan, and probably TMobile are better than ATT.
Last but not least - depending on where you live, you might have some smaller players in your region which can offer you better plans (e.g. Virgin Mobile, MetroPCS etc)