Did you not use a solicitor to help you gain your citizenship? If so they should have advised the procedure to obtain a Spanish passport. Before completing your citizenship you would have attended the embassy to return your passport - did they not advise you how to obtain a Spanish passport.
But surely because you are fluent in Spanish then you can look (google) through the Spanish government websites, to complete the procedure. It should just be a case of ticking the box in saying you are a Spanish citizen and not a Spanish national and then producing your citizenship paraphernalia. I don’t understand why it should be complicated or cost you money.I certainly don’t understand why a solicitor would charge that amount for filling out one form.
I know when you complete a UK and an Australian passport application, you have to tick boxes as to whether you are a national, citizen etc and even though we all know Spain love their red tape I would have thought the procedure would be similar.