Ideally all costs should be approved and made through the platform. You can ask in advance if they would like breakfast and then send a special offer for the revised amount. The difficulty is that some people just don't know until the day.
Because of that we looked at our pricing and now include basic breakfast items (ie: no cooked items, but bagels, preserves, tea/coffeee juice and yoghurt available).
I would guess about 50% of guests use it - some just have a glass of juice a and a yoghurt, others have more. They help themselves and we ask them to put any used items in the dishwasher. Seems to work well for us.