Just the assembled circuitboard for the O2 is $99
Buying the parts is even cheaper, but then of course you will have to do all the soldering as well.
OP: You can get a PA2V2 for $60.
Gary is an excellent guy to deal with, the NIMH AAs last a long time and it's still portable (I'm assuming you listen to IEMs while on the move), vs something the size of an O2
However, you did not specify what type of music you listen too or how you plan on using the amp, so that makes recommendations tougher :)