GIS web traffic (with ArcGIS Server): 4 people, who use 0.2 Mbps of
I can't tell exactly what you mean by this, but you are not charged for the number of times your data is requested. You're charged for storage and premium services (routing, geocodes, geo-enrichment, premium content, etc).
Vector information: 20 GB
Per Esri's Service Credits Overview page, you'll use 4,800 credits per month for vector feature services (2.4 credits per 10 MB stored per month).
Raster information (satellite imagery): 100 GB
Per the same Service Credits Overview page, you'll use 120 credits per month for tile services storage (1.2 credits per 1 GB stored per month).
You purchase a plan based on how many users you have. The plan comes with a certain number of credits, then you pay for more credits above and beyond what the plan includes at a rate of $100 per 1,000 credits.
So if you had the lowest plan you'd spend _ per year.
$2,500 annual fee for 5 users - includes the first 2,500 credits.
$2,904 for the additional 2420 credits used in a year ($242/month).
Dividing the total $5,404 annual cost by month gives you a monthly cost of $450.33.
Side note: I'm assuming the 20GB of vector features and 100GB of satellite imagery are hosted in ArcGIS Online and not on your ArcGIS Server. If you are hosting the data on your server and simply sharing it via ArcGIS Online you wouldn't use all those hosting credits.