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Any help greatly appreciated,
For an estimated max CPC you could start with
But in practice you never have to pay these max. amounts. In most cases you can do fine with 10% of the suggested CPC amounts. So a better approach might be to simply ask how much they want to spend per day and with how much per click they feel comfortable. After a while you'll learn how those clicks convert and then you can ajust the CPC per keyword to what they are worth in terms of new orders and customers.
A good start for a monthly budgtet could be the price of a full colour ad in a typical magazine they are familiar with in terms of advertising.