My advice?
Find a carpet or flooring dealer who also sells carpet cleaning equipment and consult with them. Service after the sale is important. Plus, many of them will compete with internet sales on price. Maybe not as cheap, but when it comes service costs and time to fix it, being their customer who bought a unit from them, they will take care of you. You can also ask them if they sell used units or remanufactured units at savings that might fit your budget. They may also rent out units if you only clean out a few vehicles a year that may make more economic sense (cents?) than owning a unit.
Lastly, the carpet cleaner seller may take the time to show you how to use the unit if you buy it from them and that hands-on training is invaluable. Plus having and owning your own unit means you can use it for your own house`s carpet OR do home carpet cleaning as a (small) side business.
If you "cheap it" and buy a consumer-grade carpet cleaner from Target or Walmart, while it will work, it will NOT produce the results of a commercial/professional grade extractor. Do you get what you pay for??
Ask yourself,
1) "How many cars are you doing a year?"
2) "How "clean" do you want to get them?"
3) or more importantly,"How much time can you spend cleaning a vehicle`s interior with carpet and cloth seats (like a soccer mom`s van)?"
The return-on-investment is up to you and what is important to you in your detailing endevours.