Since going online was pretty expensive in the 80s, I suppose that
   anyone that was able to afford a modem and associated online service
   costs was not concerned with the additional cost of the 850.

Nah, in '84 I got a 300 baud modem for my TRS-80 Color Computer and there were roughly 10 BBS systems that were a local call from Manchester, NH that did not cost a dime.

It turned out from Manchester you could call nearby towns like Hooksett and Bedford but not the larger nearby cities like Concord and Nashua. We had workarounds: a friend of mine developed a system with two modems where you could call into one and make an outbound call on the other and another friend has something that would set the call forwarding temporarily so you could call, hang up, call back and call a BBS. And of course you could call a BBS in Hooksett and leave messages for people in Concord.

That was if you had unlimited service, you could also get measured service where calls cost a few cents a piece. Another friend of mine tried that stunt from the movie Wargames where he called 100,000 or so numbers, ringing about 5000 phones a night. I think phone company security said "we'll let billing handle this one" and when his parents got the bill they took away his computer. But he uploaded the list to a BBS so we had it.

The public library had UNIX manuals and the Bell System Technical Manual which had articles about UNIX security vulnerabilities so it was not hard at all to find machines where the uucp user had no password and went to a shell and the like.