A Johnson or Ranco would be the next step up for a thermostat if you're currently using a rheostat (dimmer). I believe they're around $75 wired from www.reptilebasics.com
While your temps won't be as stable as a Helix you will be able to run a flexwatt set up with good success. Many people use these as a back up for their proportional thermostats, so if in the future you get more snakes and decided to pick up a helix the ranco or johnson won't be a complete waste.