I was on a comittee organising about 8 years back for the southern universities latin and ballroom competition, so not a professional one, but also not profit making.
The full day cost around £6200, that included music, demonstration couple, compare, 6 judges, food, first aiders, venue hire, officials, trophies, medals, programmes, decorations. We then charged around £12.50 entry for ~500 students.
The national competition for universities happens in blackpool every year and I imagine the costs are around 3-4x that considering the venue and number of officials. And unless we got discounts for being universities, or the judges are paid less, i imagine it costs at least £28,000 to put on a big professional competition.
In a comparable venue to the winter gardens in blackpool