The price you pay per click is determined in an automated auction, and will depend on factors like how many other businesses like yours are competing for the same clicks, available ad inventory, and consumer interest.
If you see a shift in prices, it is possible that your category and geography has become more competitive. Keep in mind, however, that you will never exceed the monthly Yelp Ads budget that you set.
For tips on how to maximize your program performance - including using keyword targeting and setting your ad radius - view the Yelp for Business blog .