That is a tough question to answer, there is far too much variation in sugarsand. It is however quite possible pump wear is causing you to use more. Just think of it like this, you may have used $100 or so worth of DE to make 900 gal of syrup. At retail that is about 2 gal worth of syrup, at wholesale bulk it is maybe 5 gal worth, still a bargain.
In the off season look into trying an pneumatic powered diaphragm pump, you will really like how they work.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.