Raj,
Exactly what are you trying to achieve/build?
You don't say whether you want this for music use or environmental studies. So assuming it is music. $100 retail is a fair price for a horn. Most of the major driver manufacturers do horns. But first of all you need to think about what sort of system you are going to build. You're first post indicates a two way, then you switch to three way. Also you need to consider what dispersion you require, both vertical and horizontal. Consider that the mid and HF patterns should match. Have a look at/listen to some off the shelf boxes like EAW, JBL, EV Community etc, and that might help your decision. You also need to consider the size of your audience. I know you said pattern control is not required, but if you are going to use more than 1 box then you have to consider it. A 120 deg horn in a box will be fine for 100 people, but that won't array properly with more boxes if you are trying to cover 500 people. Interference between horns will sound awful. The Alpha 8MR might be a little lacking in power for your volume requirements. Go up the range a little. I think P Audio do some low cost horns in the USA, so that might help. If you think you might have missed some horn manufacturers look at "Kens Links" in the PSW resources to the left of this page.
Now for the build it yourself approach. Horn design has had the reputation of being something that requires a bit of hocus pocus. So nobody will give away their secrets. The nearest you will get, is the Earl Geddes AES papers on waveguides. Have a look at his website. The Hi Fi obsessives always use wood. So if you have access to or have the skills, then wood is OK.
Iain
http://www.gedlee.com/