If by 28mm you mean anywhere from 1/48 to 1/50 to 1/56 scale? I do not believe you will find anything in a vehicle that is under $20. USD
I suppose there are cardstock print-fold-glue models that would fall under that limit. But if you are talking plastic or resin, I think you are out of luck.
If you are not in a hurry, then you can subscribe to Warlord newsletter. They run specials on a regular basis. A recent one was buy 3 get cheapest one free. If you order nothing but $30 USD tanks you bring the price down to around $22 USD (for example).
Blitzkrieg Models run a sale every now and then. Last year, every Monday, they put something on sale for 50% off. You needed to check their Facebook page every Monday to see what it was. I picked up a number of models that way (but always wanted some non-sale stuff too).
Sadly, they have not done the Monday sale yet this year.
However, even without checking Facebook, if they put something on sale, the price will be reflected in their online store:
blitzkriegminiatures.com
And Rubicon have a nice (and growing) line of plastic kits. But they are not under $20. USD