I cannot explain the 50 ft. being more expensive than the 100 ft., unless maybe it was a different brand. I mainly shop at the George Bush store in Garland, Tx., but I don't shop for 100 ft. electrical cord unless I really need it, and I never had a problem buying it off the shelf. I would call the store manager, and ask them about why Home Depot doesn't have the 100 ft. in stock in your area. It doesn't make sense that the 100 ft. electrical cord would be discontinued when there is a high demand for it.
Yes its great for up to 20 amps. We use them commerically up to that range all day long for high draw 110volt devices.