Most I have seen was £30 (or ~US$37) for the cooked or continental £24 (US$30) during a recent stay at Trump Turburry! I took breakfast as my elite welcome gift although they said on check in it was just continental but OH had something from the hot menu and we weren't charged anything extra. I have to say in defence of the charge, it was probably the freshest breakfast I have ever seen before, like the croissants and Danish pastries were just made.
Most Marriott properties are around £16.50 for the breakfast, so just about double the cost.