If you're looking to do a taillight conversion, it'll run you around $500 (pre-tax) for the full set of lights (reverse, left/right, centre peice), and headlights will run you almost $800 pre-tax for OE peices. If you can score cheap projectors, you can do a sweet HID setup, with just a little more cash.
Like Robin says, Fibreglass is cheaper, but it also is more prone to breaking, whereas polyurethane is bendable, repairable, and is the same stuff used in OE peices. I'd have to recommend poly as well (having no personal experience with kits, but hearing some bad horror stories). Keep in mind your driving skill...if you've ever accidentally gone too far in to a parking stall and scuffed up your bumper, that'd break a fibreglass one. What's worth more, spending $600 on a poly bumper, or $300 each time you need a new one?
But yeah, I think I'm done ranting for now.