Greetings! Well, it depends on how much you are running, what you want to get out of your running, and what kind of running you are doing. (For instance, if you run just a few miles a week on the treadmill at your gym, then you definitely do not have to invest a ton of money into a bunch of running gear. But if your goal is to run a big name marathon, like the Boston Marathon, or you want to become a full blown long distance trail runner, then you will be logging a lot more miles and time outside to train, which exposes you to a variety of potentially adverse weather conditions and likely some very difficult hills, trails, and running routes. Conditions like these will need some additional running gear other than just your favorite pair of shoes.)
You are right though. The amount of running gear that is available for purchase can be extremely overwhelming, and quite honestly, is not completely necessary for the majority of the time. Plus, running gear can come very specialized, and some is created for very specific runners and running styles. And on top of all that, you are right - running gear can get EXTREMELY expensive. What a runner needs to run an ultra marathon is certainly very different from what the occasional 5k-er needs. What kind of running are your friends doing? Are they getting together in groups to do long runs outside on the weekends? Are they meeting very early before work and/or school for tempo runs or speed drills at a local track?
If you want to get into long distance running, you definitely need to invest in a decent pair of shoes. (In fact, if you intend to make running apart of your typical weekly exercise routine in any amount, a good pair of running shoes is going to be worth the investment because they will help you improve as a runner and prevent injuries). If you are prone to blisters, a good, thick, padded pair of running socks can't hurt either. But after that, it comes down a bit to personal preference. For marathon training and running double digit miles, it might be a good idea to have a running belt in which you can store extra fuel and water bottles (so you don't have to carry them in your hands). And if you can't run very far without some music, you will need a holder for your phone and headphones. As far as clothing is concerned, a lot of people spend a LOT of money on very high quality running clothes. But this is one area I try and save one. I have found that generic running brands actually sell shorts and tops that hold up quite well to sweat, water, and multiple washes.