Do you have the processing ability to go 2-way active with horns? This includes the necessary crossovers
aswell as equalization....as most horns sound like *** straight out of the box.
If you want to buy used, you can get good deals on CD1 Pros and CD2s.
If you don't want to buy used, then you can just get the horn bodies from woofersetc and go to
partsexpress and get some good compression drivers. This route is typically cheaper than purchasing the ID HLCD's brand new. The Radian 450PB are a good choice for affordability and performance (used in the Illusion Audio CH1 horns). B&C has some good products aswell such as the DE500 and DE250 (B&C is actually the OEM for the compression drivers ID uses for their horns). BMS is getting good reviews aswell.
Going either route, used or DIY, you should have about $250 left to spend on mids.
This, ofcourse, assuming you have the proper xover and equalization for horns.