There's no set cost for internet and it doesn't matter whether one person uses it or several other people in your household uses the internet the internet will cost the same amount per month as long as you have Unlimited internet.
Most DSL, Cable or even FIOS internet is unlimited and doesn't have any data caps.
With DSL you might pay around $45.00 per month for internet and for cable internet it might costs $50.00 per month and for FIOS it's usually higher in costs and is usually around $80.00 per month or more per month.
You would have to look around and find an internet service provider in your location to find the true cost of the internet at your location.