Has anyone paid SpaceX for a trip to Mars on the BFR? If so, how much did they pay?
Possibly to the Moon, not to Mars.
The one possibility is the two people who paid SpaceX for the "Grey Dragon" mission, where a Falcon Heavy was going to lift a Dragon 2 with 2 tourist astronauts to a flight around the Moon. Elon has said that Falcon Heavy will not be man rated, with no mention of the Grey Dragon mission. It seems likely they might be flown on a BFR, and thus have at least a deposit paid for, but there has been no official word.
As for Mars, SpaceX has said they will give the first opportunity to sell the first seats to NASA. NASA has not purchased any tickets, nor made any indication they seem likely to do so in the short term future. Thus I think it can safely be said that no one has bought a BFR ticket to Mars.
An estimate for the first tickets to Mars on a BFR will likely be around \$200 million per ticket. A BFR mission to Mars will require at least 6 BFR flights, 12 is more likely for early manned mission (1 cargo, 1 crew), and for the first one another 6 is likely. The cost for a disposable BFR is estimated around \$350 million. Assuming that the first missions will cost \$200 million/ mission, and 12 are required for the Mars mission, then the mission will cost \$2.4 billion. SpaceX has indicated the number 12 for the first mission to Mars.
I've been doing some comparisons with the stated SpaceX plan. If you include equipment, the ticket price is likely to be something closer to \$800 million per ticket. Considering that SpaceX plans to send 4 cargo and 2 passenger BFRs to Mars for the first mission, the ticket cost just including transportation is likely closer to \$250 million. I do a more detailed breakdown in this video.