When starting out a campaign on AdWords I used the Google Keywords Tool to estimate the cost per click for each of my targeted keywords, the right hand column said "Approximate CPC (search)".
After creating the campaign and basing it on the price for both of these keywords one has doubled in price and one has tripled!
I know its an estimate, but that sounds more like a shot in the dark to me!
I spoke to a Google rep and he said the Keyword Tool prices are much less likely to fluctuate, or in other words be very inaccurate.
The best way to check out what CPC budget you should set etc. is by starting a campaign and adding the keywords to that capaign in AdWords. Bit awkward and backwards, but it's more reliable.
Might save someone the headache of having to go back to the drawing board after setting up a budget.