How much 100 GB plan cost per month?

Are you referring to iCloud?

If so, you cannot purchase a 100 GB plan. You can purchase a 50 GB or a 200 GB plan, though.

Upgrade to iCloud+ - Apple Support

What’s the difference between device storage and iCloud storage? – Apple Support

If you are talking about data bandwidth it depends on your mobile (cell) carrier options, in the UK I'm with o2 and it's £30 a month for unlimited data, calls & texts, I'm using about 1.5TB of data a month.