You can use normal soil from outdoor plant nurseries or gardening centers, its definitely cheaper but just that you'll need to check that its safe for aquarium usage (no chemicals, pesticides or critters in it) and its nutrient content is suitable for plants and livestock, if too much of the wrong nutrients are released then it might cause persistent algae issues and/or affect livestock (especially invertebrates like shrimps), of there are too little nutrients (or the wrong types) then you'll need to add base ferts and dose more water column ferts to supplement it. Generally you'll just have to do abit more work in terms of how you manage DIY soil substrates.
The main reason why aquarium soil are expensive is because its designed to be safe for aquariums and in the case of active soil substrates like ADA Amazonia aquasoil, you'll get alot of beneficial nutrients already packed in it (that are specifically designed for growing aquatic plants), along with predictable control over water parameters, so it removes most of the guesswork and produces consistent results in plant growth.
I guess it just depends on your budget... and whether you like to experiment or go for predictability.
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