I think it heavily depends on the type of plant, and what kind of life expectancy/growth you want. Some plants are extremely easy and will be fine for a long time in simple substrates. There are a few popular aquarium plants that don't even really require substrate (like java moss, java fern, hornwort/foxtail, and floating plants like duckweed or frogbit to name a few). There are quite a few aquatic plant size that rate plants by difficulty, light requirements, and substrate "richness". If you want a really lush and densely planted tank then it pays off to invest in things like enriched substrates, ferts, CO2 systems and nice lighting, but if you just want a few live plants as decoration and a Nitrate sink then gravel should be fine. You can always upgrade later.