I completely understand, but I just see the extra $20 for a stripped lower as a wise investment. I'd hate to have a $70 paperweight! Unless you buy the jig and are planning to do a few of these to make it worth your while. Same coin to me as the casual reloader who reloads 100 rounds a year. Initial investment never outweighs the benefit for an extremely long period of time.
The nice thing about the 80% to me is, that it is NOT a receiver I've wanted to buy a few to have sitting around just in case.
I agree its not a money saver. Buying and completing these will in fact cost more. For most people its not about the money but about the project. The jig is not necessary though they do make the job easier. All the measurements and specs are available online and being careful while measuring correctly they can be done without a jig. That said, I'd use a jig lol. It's to bad their jigs are said to only be 1 time use but for 15$ its acceptable.
Some are upwards of 130$.
I could buy and save money by purchasing a Nissan Versa or Chevrolet Spark (being the cheapest cars in the USA) but I choose not to. Instead I have an 4x4 Ford Exploder that I enjoy maintaining myself and a 40+ year old Shelby/Mustang worth more then some folks homes. Financially neither are wise investments. Sometimes we simply don't care if its worth it or not but look at the enjoyment we get from building something ourselves.
Question for you.. why buy a few and have them sitting around? Buy a few, dig out the drill press, mini mill, dremel, hand drill and files (ack!) and have fun making a couple.