it depends on the environment:
if the room you keep the snake in has a fairly stable temp throughout the day (i.e. not more than a 3° fluctuation) then a rheostat will suffice. i used a CHE + rheostat for kind of a long time before as the room temp was stable, but this was before i stopped using glass. a rheostat takes daily monitoring regardless. if the room temp fluctuates more than that, then a thermostat is necessary.
not to sound condescending, but why do you want to go a cheap route here? a quality thermostat is the single most important piece of equipment you will buy for your beep, and they almost pay for themselves with the protection they provide. what thermostat do you have now? and is there a reason a more expensive option is not practical for you?
i just want to fully understand your situation and offer as much help as i can. it took me almost 3 years before i bought a quality thermostat, so i understand that dropping that kinda $krilla isn't easy for everyone, but TBH i can't believe my snakes and i lasted so long without them.