Lol, Bluehost has a lot more than 15,000 MB per month of bandwidth. In fact it is 15 TB! So that's 15,000,000 MB! Or in other words, 60,000 times your current usage.
However things are not as simple as all this. It will be pretty much impossible to reach that 15 TB cap anyway. I'm not sure of exactly what Bluehost's policy is, but I can guarantee that somewhere in the fine print they will have a catch in there saying that you are not allowed to use more than a certain amount of the CPU at any time. There is no way to measure this and you wont know you have hit it until you receive an email from them saying that you are using too much CPU and will need to cut back. In extreme cases they will even cut your account off until you fix the problem.
So basically the bandwidth limits on shared hosting accounts with ridiculously high limits are meaningless. Effectively what they are saying is that they don't care how much you download because you will hit the CPU cap before you get anywhere near the data transfer limit.
If you want to know how close you are to your CPU limit, you can usually contact your host and ask them whether your site is anywhere near the limit or how close you are to it. I'm assuming Bluehost offers this service too.
The most important factor in whether your site can run on a shared host such as this, is how many page visits you receive per month and how many database queries/calculations need to be made for each visit. If you have a static site with lots of massive photos, or videos you will be able to get your data transfer quite high with only a few page visits. But if you have a data base run site with lots of server side calculations going on and small page download sizes, then that 250 MB of bandwidth could be too much for Bluehost to handle as these tend to put more load on the CPU. Big files on the other hand require very little CPU power to download them so shared hosting plans can usually handle big data transfers quite easily. I think you would be fine on the Bluehost account though. If you are unsure, just phone them up and discuss your situation with them. They will know more about what their services can handle than anyone else.
My hostgator account which costs me $9.99 per month handles 10,000 uniques per month via forums and blogs no problem at all and Hostgator has told me I am no where near pushing the limits of what my account can handle. I'm assuming those Bluehost accounts could handle similar traffic, although I assume the upper cap is probably a bit lower since they cost less.
Let us know if you have any more questions and we'll try to answer them for you
