How much would this cost?

Completely depends what type of mic you buy and where you live to how much it would cost... you could buy a microphone for $50 up to tens of thousands of dollars so yeah. Maybe ask the people on youtube which mic they have if you like the quality of theirs, or sometimes on their channel description they will list the equipment they use. Stick with someone who's not ultra famous on youtube because they'd make a lot of money of youtube partnership/google adsense and be able to afford things you can't :P