I have a 6 foot tank and the amount of skimmate I get in a week is pretty astounding. If you wanted to bypass the skimmer as you said above then yeah, I would go with 20% weekly to be sure. The skimmer can help immensely with water changes as it gets out alot of organic stuff. I keep a lightly stocked mixed reef in my 125g and do about 60% water change every few weeks. This keeps the parameters in check but not perfect. ANd again, I only have a half dozen fish right now. It also depends on the corals you keep, I have very easy ones so they are not gobbling up the elements as quick as SPS
or a montipora cap might. In my opinion it really depends on what you are keeping.
As for the expense, I go through say 200g of water a month at least, and thats just with water changes and not top offs. I have my own ro
and mix my own I/O salt so that helps alot.