I'm new to axies but not to planted tanks.
Most aquatic plants honestly do fine in just sand and don't need soil.
I've found the cheapest way is to make your own planting substrate. I do one part clay type soil (normally just found in a clean, pesticide free area), one part sandy soil, and one part vermiculite, which can be found anywhere for cheap that sells gardening stuffs). The clay has natural fertilizer built in which doesn't break down and gummy up the tank as quickly as say peat moss. The vermiculite keeps everything from compacting and sand balances everything out and keeps most aquatic plants happy. Get the whole dirt mixture damp and then add a layer of whatever sand you normally use as your substrate on top. To keep the dirty mess on the bottom make sure you add your water slowly enough or have it dispersed enough when you pour it in that the top layer of sand isn't broken through and the dirt stirred up into the tank. If you're worried that your dirt mixture will throw off your PH and what not you can soak it for a couple days and do a test. I normally cycled my tanks with a couple guppies and assumed the PH was fine if the guppies survived, I've never lost a fish because of a bad mix of planting substrate. Also most areas (in the US) will have soil content, PH and chemical analysis available through a local agriculture study. Check with libraries, colleges and the internet for an interesting read on soil break downs in your area.
^Sorry if a little bit of a rant, I've been doing planted tanks for almost ten years now.
