Thanks.
Is there a formula for calculating what his campaign will cost him?
I think Robsp is correct that the Traffic Estimator is your best bet.
You may also want to approach it from the opposite tack. Specifically, you may want to ask your client what his or her objectives are, and what they wish to spend per day as they experiment with AdWords - to see if it meets their needs.
Then, using the Traffic Estimator, build a campaign which is in line with the clients maximum spend per day. Having done this, you will then want to monitor and adjust for optimum performance.
In particular, you will want to monitor your client's return on investment. And, if you discover that your client is making money for every dollar they spend advertising, it might be wise to 'grow' the number of campaigns and Ad Groups, along with the daily budget, to fully cover their range of products or services.
AWA
I would then tell them that you will need about a week of rough estimating. Start campaigns and see how much is spent with a not to go over type budget. Then from there you will know. Some times keywords won't spend the money even if you try! So the true test will be in the live campaign.
If I knew some words or related industry I may be able to help out.
Good Luck
KG