Skateboarding is one of my favorite ways to have fun on a budget. After you buy your initial setup, you don't need to make any additional purchases to keep having fun. I've been using my same setup for three years now. I skate whenever I can, and I do it for free. Most skateparks are open to the public at no charge, and you can always just cruise around your driveway.
You can get a skateboard at a lower price if you decide to buy the skateboard pre-assembled. These are called completes. They are cheaper, but you lose the option to customize your setup to your needs. However, if you're new to skateboarding, a complete may be the way to go!
Have fun, and be safe!