If you have a mountain bike and you’re looking for a cheap dropper post for your saddle, you might want to consider a wireless dropper post. This is also a good type of dropper post to use if you have a number of different bikes and you want to be able to put the dropper post on different bikes depending on which one you’re riding that day. This KS Exaform Speed Up mountain bike seat dropper post is operated by a lever that is under the seat rather than a cable that goes to the handlebars, so the disadvantage of a wireless dropper post like this one is that you have to take one hand off of the handlebars to pull the lever when you want to lower the seat or raise the seat. If you’re the kind of person who only lowers the seat before a long descent, and then raises it up at the bottom, then it’s not a big deal. A Wireless KS Exaform Speed Up dropper post like this one is simpler, lighter, and less complicated than a cable bike seat dropper post that comes with a bunch of cables and levers. If you’re looking for a cheap simple mountain bike dropper post, you should definitely check this one out. 
Here is where you can buy a  KS Exaform Speed Up mountain bike seat dropper post:
https://amzn.to/3JZQV6R