Many English teachers may have thought about taking or not a TEFL course. The most counter-decisive fact her is the course price.
Many TEFL course providers tend to charge from 1000€ for a classical quality 120h TEFL course in Spain. If you were interested in taking additional modules like Business English teaching or Childhood teaching, that would cost you also some hundreds of euros as an extra.
There are also other type of much cheaper TEFL courses advertised online, but if the course only costs you 300€, you should not expect a real time sessions and assessments, neither the teaching practice. Unfortunately in this case you may still be rejected in some language institutions due to the fact that you lack the classroom experience.
You could be an experienced English teacher (you think that you are) offering private tutoring to your students and you wouldn't need any certificate at all - that is what you may think of. Or you could be a native English speaker offering conversational classes to your students and you wouldn't need any TEFL either - which is even worse.
Having no qualification doesn't mean having no paper only.
No matter if you are a native or native-like speaker willing to start a teaching career or you are already a teacher with some experience giving private classes, a TEFL course will structure all you knowledge into what we call a “quality English class”. Investing in TEFL course will help you to become a professional, an expert in language analysis and different methodologies. You will not need to look for worksheets or copy coursebook units, but you will be able to create your own lessons and exercises up-to-date.