Loose mineral are 'cheap', especially if you get the ones marketed to livestock producers. It is true that beat pulp has more Ca in it than P, but the amount is so low it will not correct the low Ca level of many grass hays. One would have to feed 10# or more daily and there would still be an imbalance with most grass hays.
I used to make my own loose minerals. It was a huge PITA. Then I found a livestock mineral labeled as an 'Unmedicated Wheat Pasture Mineral'. Winter weat pasture and some lush green grasses are so low in Ca and Mg that stock, especially young stock and lactating females can die without a good mineral being available. It contains 24 - 25% Ca, 25% salt, only 4-5% P, and has 2% Mg. It has the added advantage of having 150,000 IU of Vitamin A per pound in it. Horses usually eat about 2 - 4 oz. Of it daily, but I have seen horses with a deficiency eat a pound or more at a time until they get 'caught up' with their deficiency. Most new horses coming in, eat it like grain.
The mineral we use costs less than $20.00 for a 50# bag. The same mineral marketed to horse people would cost 4 or 5 times that much and probably not be as good.