This is a good topic. Sorry if I'm posting too late. I created a spreadsheet for every book I'm thinking about grading; focused on whether it's worth it if I plan to sell them. I compare what it appears I could get for the book raw based on ebay actual sales.
Then I add up all the costs associated with submitting to CCG. Besides the cost of grading at levels: Modern, Value, etc., you also have to work in CGC's handling costs and shipping costs. Their shipping costs back will kill you. It's usually good to have several items of similar level graded together to help reduce these, although insurance and declared value play big parts. The total cost charges back for shipping/insurance always seem to be significantly higher than when I ship a book with Insurance.
Anyway, then I take the amount I think I can get for the graded book (assuming I'm anywhere close to what they come back with) from ebay sales again, subtract all the CGC charges, and compare that to the expected raw price. If it's not significantly more, it's not worth the pain and effort.
A big variable is what grade the book gets. I did several test submissions of 7 or eight books to CGC in the early days, and nearly always was only a half-grade off up or down, so I'm pretty confident in my grading. But lately it's changed. Once you get over 9.2 it gets extremely tricky, and the values wildly fluctuate. Lately I've seen already-graded books at cons with 9.6 / 9.8 grades that have flaws I can see that would have me grade it 9.2 or lower.
Anyway, sorry for the long post. Just describing how I try to cope with the decision. If you're not slabbing to sell, the analysis is likely very different. Good luck.