I currently have a 1100 gallon tank. It cost just under $400. Around August the tank does get low but I've never run out. There are tank systems that go 10,000+ gallons.. It all depends on how much you want to spend (and how much water you need). I cobbled my system together on my own, so it was pretty cheap. I've got under $1000 in the whole system.
So long as you don't hook into your house water system (especially if you're on city water), these systems are very easy to set up. If you're on city water, there are usually codes that make it difficult/prohibitive. They're concerned about you "back flushing" your rainwater into the local water supply (and your rain water ending up in your neighbors house). But if you're doing irrigation only, there are usually no codes.
All my system is, is roof water drained into a big tank. There are two screens for filtering (mostly for leaves and dirt). A big screen then a smaller one. There is a small circulating pump inside the tank that keeps water circulating and helps to reduce algae. This isn't really a big deal as I don't drink the water. The tank is sealed and not in direct sunlight, so mosquitoes/etc are not a problem. The circulating pump is run off of two solar panels I set up. Finally, there's a slightly larger pump that runs the irrigation system, all on a timer.
It's also handy if you live in an area where they limit the days you can water on. When you're on a system like this, there really aren't any limitations.
If my tank ever ran out of water, I could, if I absolutely had to, put some water in the tank from a house water hose. I would do this only because the irrigation system is a closed system and it would just be easier. The irrigation lines, etc are all fed off the rain water tank. But my tank has never ran out of water... yet