I am currently visiting Spain, and just left Madrid. Here is some news others may find useful:
Even though museum prices are in Euros, I think the museums are really "dollar based." For instance, the entrance fee to the Royal Palace in Madrid is supposed to be 10 Euros, but with the fall of the Euro, they raised the admission to 11 Euros (a crude stick on is pasted over all the signs). Many years ago when I was in Euro during a dollar rally, I noticed this same thing, with admission prices rising to keep the price the same in dollars. Museums and tourist sites want the dollar's buying power, even though their prices are in Euros.
According to Rick's 2015 Spain ebook, the admission to the Centro de Arte Reina Sofia for over 65 year olds (me) is supposed to be 3 Euros. I was pleasantly surprised when they told me it was FREE. I hope this wasn't a Holy Week anomaly.
Finally, Rick's book gives many long line avoiding tips for visiting the Prado. One I found was buying the tickets from the concierge at my hotel (The Liabeny - one of Rick's recommended hotels). They stock Pado tickets (and other tickets) and charge you the same price as you would pay online or in person. We bought our tickets from the concierge about 15 minutes before going, and just waltzed right in, whereas the lines for picking up pre-orders was long, and for people actually buying tickets was super super long. I figure we saved an hour by buying from the concierge, and to the best of my knowledge the price was the same as if we bought online.