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One of the hosts I use charges £2/month ($3.54/month) per gig if I exceeded the monthly transfer limit. Another host I use charges two-three times that amount per additional gig with a very small basic transfer limit.
When purchasing hosting always check bandwidth limits and additional cost per gig - if their pricing says $5 per gig then that's how much they are going to charge because that's what you're agreeing to by using their service.
If your host doesn't provide the email warning service then maybe you should consider switching, especially if you plan on going over at times.
They also send a threshold notification that you can set at any percentage. So you can set it to email you when you reach 80% of your bandwidth usage...
Same thing with diskspace. They'll let you have as much diskspace as you want but if you reach the limit of your plan you can be notified if you are getting close. This feature alone saved my butt one month when one of my clients uploaded a bunch of psd files instead of the flattened jpg files.
I suggest you look at dedicated server providers that include a high bandwidth allowance. There are several that provide 700 to 900 gigs per month for under $150.
If you go over these allowances the typical cost seems to be about $3 per gig, but before that stage you should definately be adding additional servers so that the overage charge never actually occurs.