Log it to a table then just compare the last reading with that for the previous week.
========================================== Cursors are useful if you don't know sql. SSIS can be used in a similar way. Beer is not cold and it isn't fizzy.
If you do backups on a regular schedule, just query the tables in msdb to calculate the deltas. This will provide you information on their growth rate. This way you're not reinventing the wheel (logging to a table) and you just measure the difference between your full backups. If you have differentials in mix that provides the rate of change to the granularity of whatever schedule your diffs are on.