I'd say one of the OS7 Curves or even the all touch Torchs.
I always seem them on Craigslist for $125-$175. The Bolds I usually see on there range from $200-$275.
Although I did see one for $125 but the back was all scratched.
The 9810 seem to be the best bang for the buck when bought used. The curves are only slightly cheaper, but not in the same league as the 9810.
I just bought a used 9860 and it is pretty snappy, maybe even quicker than the 9810 for scrolling web pages or just navigating the OS screens. The lack of a keyboard could be a problem if you do a lot of long emails on it. My son is using it mainly for BBM, texting, Facebook and etc. He seems to be happy with it.
Basically, you can also find cheaper pricing for other OS7 used Blackberry phones and then you can unlock your Blackberry and use it with a SIM card from AT&T
I was very impressed with it. I just get the flagship BlackBerry but a friend of mine wanted to get into smartphones so we went and looked. She liked the size, look and feel of the Curve.
What surprised me was that, after helping her set it up, I loved it too. Not quite enough to give up my Bold to be sure but I was very impressed.
Not only was it super fast and it had FM Radio capability (no data charge - which is another reason ATT is dragging their feet on 7.1 IMO) but in a time when everyone is going bigger and bigger on phones, to the point you can't even put them in your pocket anymore, this little guy is going for smaller, slimmer and just awesome.
There is no touch screen but other than that it is all OS7. NFC, fast browser, just awesome. Yeah, and for 29.99 it is just a deal.