It's easier to trace the roots of this if you look at paying not "for your half" but "for your pipe to the rest of the network".

UUCP mail and USENET newsgroups often worked this way. A site already on the network would allow others to phone it (at their own expense) and thereby join the network. The original nonprofit UUNET, a telephone UUCP/USENET service, was designed to reduce these phone costs, particularly for people who had to call long-distance, by aggregating the calls into one company, getting a volume discount, and charging at a rate designed to recover the costs. It worked.

When three companies started the second Bay Area regional ISP, The Little Garden, in 1990, we split down our $1000/mo UUNET Internet bill evenly, plus, each of us also paid the cost of our own 56K leased-line (or dialup modem-line) that tied us into the nearest point on the network. After connecting about thirty individuals and companies, things got more formal, with these costs more buried in generic pricing. TLG became the backbone of many dozens of little ISPs in N. Calif.