They run for about $30 average on Ebay...here's a BLACK CHROME one for $45:
I thought about this same thing yesterday, as there are many times I wish I had a directional spot light, plus it looks cool in my opinion. All that's involved with mounting them is drilling a hole in the a-pillar, mounting the bracket with 2 screws, and securing the wires. Since the a-pillar trim snaps off, and the fuse-box is directly below it, this would be a SIMPLE modification! I may have to do this one, if my wife lets me, of course!