Cheapest way to fix seats with tears and missing foam?

I don't know how "cheap" you're thinking. New drivers seat bottom foam bun for my 2010 RAM was only about 28 bucks @ ebay. I don't know that its large enough to be trimmed here and there to fit your car though. The OEM correct seat cover was a bit higher at 178.
Honestly, browsing the leather seat skins available at TEMU website is enough to make me want to sell my OEM 67 seats and head to the parts yard for seats those skins would fit.