When sending targeted campaigns through Market Locator you are subject to an contrained number of available clients with a limited number of messages that can be sent to them per month. Market Locator uses bidding to determine who's message is sent.
Campaign budget and campaign bid
When creating a campaign, you not only decide how much you want up on the campaign as a whole (the budget for the campaign), but also how much you are willing to spend as a maximum for a single delivered message (the max bid).
Second price auctions
Market Locator uses second price auctions to determine who's message will be sent (similarly to Google or Foursquare). It is important to note that most of the messages you send, will be sent at the minimum price, only those which are targeted at by multiple advertisers enter into the auction. The highest bidder wins that specific client and pays the second highest price (for the economic theory curious clients, we are using "second price auctions").
TIP: If you want to be sure that you really reach the required number of people, set the maximum bid for a higher amount. In most cases you will anyway pay much less.