The other option available to you is a monthly transfer limit, an "all you can eat'' of the ISP's bandwidth within that limit. Depending on the type of site, this may work out to be less expensive. For example, if you have a huge surge of users at 6 p.m. but a lot of dead time elsewhere, this option could be for you.
To calculate your monthly transfer requirement, simply multiply the number of monthly user sessions you expect to receive by the amount of data each will transfer.
If a typical session consists of 10 requests and each request has a page weight of 50K, each user session will consume about a half-megabyte of transfer. In the earlier example of 5,000 regular users each using the system once a day, this represents 5,000 sessions per day, or 2.5 gigabytes of transfer. During a month, this works out to 75 GB/month.
Ask your ISP which is likely to be cheaper 75 GB/month or 4 Mbit CIR.