There is no minimum spending requirement; the amount that you pay for AdWords is completely up to you. You can, for instance, set a daily budget of five pounds and a maximum cost of ten pence for each click on your ad.
Wiser Marketing provide keyword traffic and cost estimates so that you can make informed decisions about choosing keywords and maximising your budget. (Estimate keyword costs)
You are charged only if someone clicks your ad, not when your ad is displayed.