Monterey is cheaper than Carmel. Carmel has a more exclusive scene, with bed and breakfasts or rentals, while Monterey offers hostels and motels. Monterey also has more affordable restaurants.
If you want to see Carmel-by-the-Sea but like the prices of Monterey better, stay in Monterey and drive down to Carmel village for the day. It’s only a 12-minute drive south.