Just an FYI, but most of the time mods don't raise the price because you limit your possible buyer pool since not everyone has the same taste. That's why whenever I mod my cars or my bikes, I always keep the stock parts. When its time to sell, I put back what I can and sell the parts separately.
That being said, I usually do a comparison for my area between NADA Guides, KBB and Edmonds and average out the three. I find KBB to be pretty accurate.