Dentists in Spain will always charge you even if you work here and pay social security contributions. One way to cut on your dentists bill is to have adequate dental insurance which can be purchased from the main insurers atarting at as little as 10€ a month. Although the insurance will not cover the whole bill, it can reduce it by up to 50%.
We suggest either you go to a dentist a friend personally recommends, or you go for a English speaking dentist. There are even some english spoken dentists who offer the first consultation for free. Happy drilling!