This is what wikipedia says regarding EC261.
If a passenger is placed in a lower class than that for which a ticket was purchased, the airline must refund 30/50/75% of the cost of the ticket for type 1/2/3 flights. (...) It is unclear whether the refund relates to the price of the entire ticket, or the segment on which the passenger was downgraded.
I am not sure if EC261 applies, but I suspect it might because the dstination was in Europe, and you bought the ticket from an EU airline. Your flight is type 3, so 75% of the ticket price, not the difference in price? I never used this provision, and I have not checked or the details, but someone here would know whether EC261 applies or not.