Calculating bandwidth can be a little challenging
You don't want to little as this can doom the project from day one. Too much a you are spending precious capital on something that may never be used.
As a rule it is better the go more than less. Here as some tips and figures to help you make the calculation.
Website bandwidth and compacity are finite
Yes finite, even that $89 Unlimited bandwidth plan from Godaddy has a limit. The limit may not be download/upload bandwidth, but rather compacity, to say the maximum amount of data allowed through the port at any one time.
You need to make sure you have the bandwidth down/up for http transfer and if your in development you need to know whether ftp transfer is included in the transfer limit as well.
Every item that displays on a website page has a cost of delivery and that cost is measured in its size 'on transfer'. The size is measured in kb pictures and videos need to be the more costly items and code and text the least. Consequently when someone views your page the some total of all the files on that page will cost X kb in bandwidth to transfer.
So the formula to calculate bandwidth requirements is
Page/website size X total monthly views
Compacity calculation is
Page/website size X Max simultaneous users
bandwidth isn't to tricky compacity can get very tricky, when you start to factor in different Locations, max users per location/s.
A confounding factor to all this can be latency as well. If your server is located on the other side of the world and taking 3x longer to send the data the compacity is effected even though total simultaneous users and data usage is the same.
What every you do pay more for flexiblity
Unless your a 100% sure on your requirements for the next two or three years. Pay the extra and go somewhere that will expand or shrink your plan as required. Migrating to another server is a big expense and hassle.