Have you taken a look at netstat? It is part of the default install and allows you to see how many bytes in and out of an interface.
Example: netstat -I xl0 -b
Note: I do not know at what point the numbers roll over (if ever) or if there is a way to zero the count after you read it. I am currently at a windows machine and unable to test this.
It is a start at the very least.
Edit: You might also be interested in
Which appears similar to the old tool you were using.