I did the same thing with a
Jesus fish @
SuperChrist.com
Depending on how large you went, I'd guess the dye would cost around $50 (if you went the "986S" route -"Boxster" or "Speedster" would require a considerably larger dye). The fun question is, what kind of material would you be using for the letters? There's brushed aluminum that may look good. I, myself, enjoy anything reflective. Since I love monochromatic schemes, reflective color to match the color of your car would be really cool. Plus, at night with headlights blazing your car's rear, the logo/emblem would REALLY stand out.
Send me a PM if you want the info of the guy who did my dye or to see photos of how the the reflective shows up at night.