In theory what you say will work. I am totally on ambient temps in my snake room. The eggs still go in the incubator though. Even if heat does not cut on that often temperature and humidity fluctuations are easier to control that way. In the room I keep temps between 84 and 88 in general. It is my belief that 89 is pushing it. I have seen the beginnings of heat related stress at 89. I also would not recommend bumping the humidity up in the room as a whole unless you live in a very low humidity environment. High humidity plus little air circulation promotes mold growth in the room itself. The tubs themselves should boost humidity to the proper levels. As to cost, the few months my heat is actually on it is about a $20 increase in my electric bill.