The most cost-prohibitive part of running in the USA is medical injury. With the dismal state of USA health insurance, a couple of visits to an ortho and a MRI can exceed several hundred dollars in co-pays and deductables.
The next biggest cost is food. 75 miles a week is about 7,500 extra kcalories or 15 big Macs. I eat better and cheaper than that, but figure at least $20 extra food a week. Thats going to be several times the cost of wearing-out shoes.