Some fellows here will tell you you can record loud rock concerts using 4.7k modded microphones plugged directly into mic-in (or was it line-in ?) of the Edirol.
Doing this way, you can spend about 300$ on a pair of cardioid (uni-directional) mics and send them to Chris Church who will do the mod for you. No need for an additionnal battery-box or preamp.
If you want to use omnidirectional mics, you'll need a switchable bass roll-off filter on either a battery-box or preamp (assuming the low-cut filter option on the Edirol is not enough for loud rock concerts, but this has to be discussed...).
If you don't want to use a battery-box or preamp only because of the extra cost, I'd suggest you to check Church Audio products too !!
You could grab a nice sounding pair of CA-14s and a great CA-9100 preamp for less than 300$ (check the retail space of this board).
If you don't want to use a battery-box or preamp because you don't want to bother about it while at the show, I'd suggest you a pair of 300$ mics + 4.7k mod.
A 300$ budget for a pair of mics will let you a large choice between some of the best stealth products out there (SP-CMC-8, MM-HLSC-1).