I love Herpstat. I will not recommend anything cheap and think you should buy the best tstat you can afford as its the most important thing you will buy for your snakes. Make sure you think long and hard about your plans and what you want your collection to be. It might be cheaper in the long run to buy a multiple port tstat now. http://spyderrobotics.com/home/products.html
Many also like these options. http://www.reptilebasics.com/thermostats
Use the search function and you will find these is a very commonly discussed subject.