Don't try to home-brew your own coolant from scratch, that's setting yourself up for disappointment. There are other ways to save money here while still getting quality mill coolant, though. Cutting fluids/coolants from conventional CNC machining are available for far, far cheaper than their dental equivalents, but (for the most part) machining is machining. You can do some investigatory work and compare the MSDS sheets for the manufacturer-recommended dental mill coolant and from an affordable general-purpose coolant from a machinist's supply place, you'll probably find they're mostly the same stuff, even if they don't tell you the exact proportions.
Caveat: I don't know if the ceramic n other oddball materials dentists work with have some special cutting fluid requirements, but metal milling is going to be more or less the same across industries, so I'd expect broad cross-compatibility there. Just do your homework before going and putting non-OEM coolants through your mill!