5 gallons sounds like a bigger saw than I'm used to. I have a 12" saw that only uses 2 gallons. You can get four gallons of mineral from Santa Cruz vet for about $50, shipping included. If that still blows the budget, I would at the very least treat the water with something that prevents corrosion. Anti-freeze that is used for storing the potable water systems in an RV is very cheap and does a good job. A gallon mixed with 4 gallons of water should give you a good enough solution to protect the moving parts. WARNING Note!!! it's NOT the anti-freeze that you put in your radiator... very different stuff, and very toxic.
Now for another suggestion for when you get a few bucks to spend...
Up until about two years ago, I used nothing but mineral oil in my LS-12. However, I had acquired some chrysocolla that needed to be stabilized and I didn't wanted it absorbing oil, so I did some R&D on water-based cutting juice. I landed on this stuff called CRL Diamond Tool Coolant. It's a water soluble concentrate that you can mix two fluid ounces per gallon of water. Not only did it have a corrosion inhibitor, but it provided lubricity that I never could find with other water-based lapidary cutting fluids. My saw runs cool and I haven't noticed any difference in wear on the blade from my mineral oil days. It's environmentally friendly and can be disposed of easily. Also, my saw clean-up isn't the ordeal it once was. I've cut all kinds of material since switching to CRL and don't ever see myself going back to horse lax. You can order it on Amazon. I was paying about $20/quart (incl. shipping) but I recently saw a price increase of about $5/quart.