percentage clickthroughs

impossible to answer but guidance needed

[topr8]

have not been involved with overture before but have a client who wants to get involved with ppc.

i know its an impossible question and title/description is a factor, but is there a rough guestimate of the percentage click throughs on a keyword??

we want to have a vague idea of the possible cost of a campaign. (i realise we could set a monthly limit to cap the cost)

so for instance if a given keyword is searched 10,000 times a month according to the keyword tool, how many ball park hits would a top 3 position expect to pick up???

any ideas???