it is telecom companies and internet companies that determines the cost of internet usage.
I cant tell how much data you will consume in month by browsing or watching videos.
Different companies have different internet products.
Also I dont know what network have the best reception in your area .
Vodafone provides 10GB of data for 30 days which cost 100ghs
Airtel provides 700MB of data per day which cost 1ghc
Glo provides 30GB of DATA for 90 days which cost 80ghs
MTN, tigo and surfline all have their subscription too
Wifi allows more people and devices to connect to the internet while modem can only be plugged into one device at a time.