
I use Google Adwords to promote my company. You can specify the maximum amount of money you want to pay as well as what you want to bid per click. So you can say you only want to spend £5 a day for example. When you've reached 5 pounds your ad will not be shown anymore. You don't always end up paying your bid price either, sometimes it's less.
I can't tell you about Yahoo as Google was providing about 70% of all our traffic before the campaign so I only went with them as a trial.