Worth Grading
Grading Ditto (151) should cover its own fees comfortably. Raw copies trade around $0.39 while PSA 10s sell for about $163.50, a 81% Return on Grading. Condition still decides the outcome, so only send copies you'd back for a 10.
Among the 78 cards in 151 with PSA 10 sale data, Ditto ranks 26th for Return on Grading and 30th by PSA 10 price. Of the 13 Rare cards with PSA 10 sales, it is 6th for Return on Grading. The median PSA 10 in this set sells for $131.00. 19 of the 78 clear a 100% Return on Grading.