It all depends on how much you are willing to spend. For example at Lukads you enter a maximum amount that you want to pay per 1000 impressions or per click and you won't be paying more than that. So for example if you set the maximum at 3 cents per click, you will be paying 3 cents or less for each click that your advertisement receives.
How long is a piece of string? That is how vague your question is. There are many variables to consider such as traffic and relevance.
it depends on what niche you are targeting and what type of ads and audience you looking for like PPC , CPM , Layer Ads, CPV, CPA and each one have different cost and different bids
as i know its depend on ad size country traffic target and many other also its depend on your budget take a Google ad word tour you can find more knowledge