Here in on the west coast of central Florida, a rehub will cost you $60. If the prop will already fit your propshaft, you don't need a rehub, just the right thrust washer, and propeller nut and washer kit.
Based upon the rpms you're getting now with the 21P, I'd venture that the 17P is about right.
As for price of a used prop, you can't do much better than the $70 you mentioned.
What I want to know is where to you put all 30 of those women, and if they're midgets or something
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Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury