This is my rainwater collecting barrel that I made to water my garden. It cost me about $50 to make but you could do it for a lot less. Store bought they are closer to $100 or more for a good sturdy one. It was very easy to make.
First you need about a 55 gallon food grade barrel. You can get these free from soda bottling companies. They fill them with syrup and then throw them away. I had no luck with this option and got mine for $40 on ebay. The hole on the top for the water to come in has to be mosquito proof. I took an old piece of screen and attached it over the hole in the top with a large hose clamp. Then I drilled a hole on the side at the bottom for the spigot. The spigot cost about $4. It was threaded so I just screwed it into the hole I drilled. If you warm it up a little it helps it go smoother. It took a few tries for me to get it to go in straight too. Once it was in place nice and straight I sealed it with waterproof caulk all the way around the edge of the spigot. A big tube of the caulk cost about $4. Done.