What about over-the-counter teeth whitening kits?
While the whitening kits you can find at your local drugstore cost less than professional kits, the truth is that this is a case where you get what you pay for.
Drugstore whitening kits are designed to be "one size fits all," which often translates to "one size fits none." The strengths are much lower than what you can get in a professional kit, meaning that you must use the product for a longer period of time and are still unlikely to get the results you can find in a professional product. Additionally, your results are not being monitored by the dentist you know and trust.