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It depends on what you mean by planted tank. You can grow lots of plants in plain gravel, under a T8 shop light fixture. You can use a mineralized soil under gravel/sand substrate for even more possibilities. A basic setup can be done for under $100 in a tank that size, assuming you get the tank for a buck a gallon.
On my 45 gallon that I just started I think I'm probably up to 600-700 bucks, but I went with an LED light and fancy substrate, etc etc. Ironically I think the cheapest part of the whole thing so far has been the plants!