Internet in Spain is expensive however If it is a Telefonica line then it is possible to move to another provider where you will get a lower cost but also a lower speed
There are a number of alternate providers and all you need to do it contact the provider with the telephone number and they can advise if this is possible to manage and move the line, and providers include people like Europa, Round Town News Networks, Citrus Red, Bubble
The other thing is that the dongles do work well but obviously service cover varies so check what mobile network works best in your area, the one with the best signal strength and then normally that will provide the best internet access