The starter sets you can buy from eBay are very economical. There are different flavors of boards, tailored to what you want to do with them. The two developer boards that are the most common are the uno and the mega. They work very close to the same as each other. The main difference is that the mega has more memory and interface pins to work with, while the uno is much smaller. Either can be had with a proto board, a handful of jumper wires, and a few dozen discrete parts and hardware for under 30 bucks shipped.
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