Your best bet in terms of brightness and cost is by using cathodes. Cathodes are much brighter than any liteglow,streetglow,optx, etc 15" neon tube.
You will be blown away by how bright they are. Just one set of cathodes (comes with 2 12" tubes and a transformer) will light up your whole grill and more!
Cathodes are much cheaper than any neon tube as well. I sell one kit for $6.49 each. You can get one kit shiped for under ten bucks.
And yes... Iceled is a freakin rave under your car. I've spent many hours at night just stairing at it.