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I just did one with a dirt and a play sand cap. Dirt was $13 (miracle grow organic potting mix) enough for 2 tanks and a $2.50 for the sand I have left over on that as well. My last one I did with an off brand organic dirt for $6 and just reused the gravel that was in there before.
You can use a lot of different substrates in planted aquariums, dirt, sand, fluorite. I have never personally used dirt, but I hear it is one of the best (and fairly cheap!). I use fluorite in my tanks and it works very well, and it is not to expensive.
The choice of substrate really depends on what you want out of a planted tank.
If you are going low light then you can settle with plain gravel or sand and do just fine.
If you want medium light low tech then sand with root tabs or a plant substrate such as eco complete or flourite (both of which will need root tabs) will work just fine for you.
If you want medium or high light high tech then I would highly suggest either dirting or a plant specific substrate or both. Eco Complete, flourite, or aqua soil are good choices. Root tabs will still likely be needed.