One time some guy want sell me Senior Member account for 0.05 !!! but he talk something that there is "potential activity" Sr. Member.. I thought that was suspicious cheap and gave up.
Potential activity accounts do go for much less because you often need to make a lot of posts to collect the full activity. For instance you could have an account with 60 posts/activity but a potential activity of 240 so you would need to get your post count up to 240 to get all the banked activity which can be a bit of an effort.
so verdict of price
This is probably the most accurate estimation:
Now I see them go for as little as 0.15 but lets say between 0.15 and 0.25.
If you want an account just check in Digital Goods or make a thread there wanting to buy one and see what offers you get.