In the US, it depends on the provider. The two giant cable telecoms I've dealt with charged ~$5/month for a wifi router which is really highway robbery - especially considering the quality of the routers they provided. Their provided cablemodems were pretty much universally awful too, so you were better off buying both your own modem and router - but they obviously didn't advertise that you could do this, if you weren't technically savvy you'd never have a clue. One friend I helped out was amazed that they could get wifi without paying TW $5/month and how much better it worked without TW's equipment.
I'm currently on Qwest (formerly anyway, now acquired by CenturyLink) DSL. Qwest provided an excellent modem/wifi router for free, but I have no idea if CenturyLink would do the same now - I kind of suspect the giant teleco would probably provide inferior equipment and/or charge for it.