If you're referring to DSL/Dial Up companies, the infrastructure was already in place for that. Phone lines have been around for decades. Anyone can start a DSL/dial up company, but what good is that? The technology is outdated.
To be competitive in today's market you need to be running fiber. Depending on where you're located, fiber is scarce and running it isn't cheap.
No. It always took lots of money. Maybe not for dial-up, but otherwise it's always taken a lot of equipment, not to mention the tearing up of the roads to lay down lines.
DSL uses, in most cases, the same copper phone lines as dial-up. So unless you're deploying a cable network (which has multiple revenue streams if you're selling TV, thus a higher market penetration rate) or a fiber network, there's no need to dig up roads or lay your own lines.
Yeah, the equipment was probably expensive, but compared to what? Laying your own infrastructure? Not even close.
I suspect that you're already aware of this, given that you run an ISP, but for those who are not familiar ... the reason that there was a proliferation of DSL-based ISPs in the late 90s and early 2000s, such that for a brief moment there was actual competition in the broadband market (in some areas), was due to utility-style regulation by the FCC. Specifically it was called "local loop unbundling" and it required that the incumbent phone system operators lease "local loops" (the copper that runs to your house) out to independent operators at reasonable-and-non-discriminatory rates.
This regulation was undone under the Bush administration and most of the independent DSL operators died shortly thereafter, leaving consumers with a choice between the incumbent telephone company and the incumbent cable company, in most cases, for broadband.
In short, when the market was regulated, there was competition and consumer choice. When the regulation was removed in response to industry lobbying efforts, consumer choice disappeared.
I'm not sure if you know the answer to this or not, but how did they deal with capacity? Were you leasing the line or were you just leasing X capacity? I think that's the biggest issue with cable companies right now. Most of their infrastructure can't handle the amount of data they're trying to pipe through it, so I would think regulating it would only saturate it more.