For a really REALLY cheap setup, I would go the Walmart route. If you can't afford the 10 gallon complete setup, just buy the 10 gallon tank. Then, when you have a few more bucks pick up a heater. Then, when you have a few more bucks, pick up a filter, etc. As someone who started out fishkeeping with nearly no spending money at all in college, I have learned how to buy things you can't afford: One piece at a time.
If you think a grand total of about $50 to $100 for a planted 10 gallon, broken up into chunks of, say, $10 purchases is still too much, then you could always check garage sales and online giveaways as a possibility. Otherwise, you are left with nothing but a betta in a vase or little cup as an alternative.
Two 1 GA desktops, a 5 gallon, a 2.5 gallon, a 10 gallon, a 20 gallon, a 25 gallon, and two 75 gallon tanks, all planted.
I am an expert on algae, so ask me if you have questions. I know how to grow it, just not how to get rid of it.