i am a owner of a 08 bullitt.
in my OPINION the bullitts are worth a little bit, not really that much, more than a GT. maybe $2-3K over comparable GT. keep in mind the bullitt option was $3,310 over a premium GT. my car had a msrp of $32,370 with a shaker 1000 radio and no other options when new.
will they be a collectable worth big $$ over a GT 30 years later?
in my opinion not really. yes they are limited production, 7,000 cars built over 2 years, but not that limited to be worth a significant amount over the same condition and miles GT.
they were originally built with a special muffler and H pipe for a different sound and got a different tune so it has about 15hp more than a GT. they also had a exclusive dash trim, grill, sill plates, trunk emblem, special colored wheels and brakes, seat covers like a GT500 without the snake on them, and rear spoiler delete. i THINK they also were about 1/2" lower than a GT.
if you really look around you can find them in average condition with average miles for the year for low teens. in my opinion for $25K it should have EXTREMELY low miles and be in mint condition.
also my opinion look for a high miles 07 GT500. they can be found in the low $20's with some miles on them and the mods will be already done at the factory. add a pulley swap, tune you will have more HP per $$$ than you ever hoped to get with modding a GT or a bullitt.
hope this info helps you decide.