People who do not wish to buy a GTLD can use home.arpa as it is already reserved. If you are at the point of setting up your own DNS but cannot afford $15 a year AND cannot use home.arpa I’d be questioning purchasing decisions. Hell, you can always use sub-domains in home.arpa if you need multiple unique namespaces in a single private network.
Basically, if you’re a business in a developed country or maybe developing country, you can afford the domain and would probably spend more money on IT hours working around using non-GTLDs than $15 a year.