You could do a conductivity sensor probe or an ultrasonic sensor but there's no way that either of those would be less expensive than a float switch because they're not intrinsically discrete devices like the float switch so you would have to have some sort of signal transmitter where the user could program in the desired setpoint.

Another option would be to cantilever a platform onto a load cell and then calculate the weight of fluid at the height where you want the heaters to energize. This too brings you back to the problem that the load cell is going to transmit a continuous signal and you will need some sort of controller to program in a setpoint.

From what I've seen there is little technology that is more economic or more reliable than a float switch. It's basically the bobber you have in your toilet that manually closes a relay completing a circuit. The only reason we start looking into alternatives is when the fluid can't come in contact with the sensor (use ultrasonic), there's not enough physical space to accommodate a float (use conductivity probe) or we need continuous level monitoring.

The application you are describing sounds almost analogous to a sump pumping control system. The most basic of these systems often utilizes a series of float switches to annunciation the following signals (installed from the bottom of the tank/pit going up): pump off, pump on, alarm high (activates a visual our audible alarm), alarm high high (shuts off transfer pump or closes a influent valve). If you need more information on how to control your system just Google "sump pump control panel, sensors" and you should find a wealth of information very closely related to your application.

Again, unless you have a corrosive liquid or a sludge/slurry I can't think of a more reliable level sensor than a float switch.If you're not familiar with McMaster Carr I suggest you check them out (I linked you to the level switches page). Visiting that website is like going to the Lego store.