ACID Xpress is a freeware version. Very limited, gives you a taste of what can be done.
ACID Techno and ACID DJ are the next level; I'm not quite sure what the difference between the versions is, they seem to be parallel versions, maybe just packaged with a "techno" or "dj" set of loops. They sell for about $30 US.
ACID Music 3.0 is the next step up and sells for around $60 US.
Finally we get to ACID Pro, which sells for about $400 US.
Here's a comparison chart at Sonic Foundry's website...