What kind of substrate are you using? Co2? Lights?
With 90 gallons, yea dosing is going to be expensive. I have no found any liquid ferts to be more cost effective. The manufacturers want to make money so maybe experiment a little? You could probably get away with 75% of the recommendation.
Personally I use an inert substrate and find root tabs to be more cost effective.
However, while I don’t like mixing substrates, you could always layer yours with soil.