If you have time before setting up the tank, then I would definitely consider using mineralized topsoil, capped with something like turface. Mineralized soil is an amazing substrate, full of everything your plants could want, and it's free except for a bit of your time. There's plenty of information about it on this site if you do a quick search. It's hard to get much cheaper than free.
That being said, if you're set on something tried and true, and made specifically for an aquarium, then many LFS carry specialized substrate like Eco-Complete, Flourite, etc. They're all about the same price though, in my experience, give or take a couple bucks.
Feeling adventurous? Maybe check out my current
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