I spent around $1200 (Canadian) on a bike, helmet, lock, rack, panniers, straps, tubes, bottle holders and a few other odds and ends. You could easily do it for less than that if you searched around for second hand stuff, or if you have access to a bunch of tools and materials to DIY things like panniers. If you already have an existing bike, you can start with that instead of buying a new bike. (I actually bought my bike about a year before the rest of the stuff.) Just make sure you get a sturdy rack and be prepared to replace the back wheel with something hardier in case it can't take the weight.
For me personally it was worth it to buy stuff new, because i hate the tedious waiting around and hunting and bargaining involved in trying to get a cheaper deal through other channels. I'm happy working in a regular job to earn more money and then paying 2x the price for the luxury of being able to walk into a store (or log on to a website) and get something in good condition right then and there. Your mileage may vary.