Cool books, but remember that since those come from different eras and have different values, you will be stuck making several different orders. meaning more costs involved for shipping and the like.
So keep the total cost in mind when you are deciding what to send. As for the answer to your question, I feel that if the market value for the slabbed book you get is at least double your total cost (meaning what your book cost, what the cost of slabbing is, shipping both ways), then it's good. I see people saying numbers like 300$ being a cutoff, but an arbitrary cutoff like that is meaningless if you ignore your total cost (granted, they were simply answering your exact question). If a book cost you $250 already and it would be worth $300 slabbed, then grading it is only gonna make you lose money.
And when thinking of market value, be conservative on your grading because CGC can be tighter than a puritan girl on her wedding night.