A post on Windows Mobile data usage by Jason Langridge's WebLog - MR Mobile!
Exchange 2003 (Pre-Service Pack 2)
From working with customers and looking at their data usage the average device used:
Smartphone - 4MB
Pocket PC Phone - 6MB
Pocket PC Phone was higher because most users tended to download more attachments.
These numbers aren't too scary as you can get a very cheap data plan for that kind of data usage. The highest I've ever hit on my device was 30MB when I was doing lots of demos of streaming media.
These numbers were all without any form of compression!
Exchange 2003 with Service Pack 2 and Windows Mobile 5
In SP2 we've added data compresssion to the Activesync solution which results in about 50-60% compression of email and 30-40% of compression for other items such as calendar, contacts and tasks. Overall the compression rate is about 40-50% in my personal experience.