A 7x5 digital print costs about 5p
First, take the salary the photographer needs to earn to keep body & soul together. Let's be modest and call that £20k.
Add on the annualised cost of their kit, which in the days when most pro photographers replace their bodies every two years to keep up with developments, is a lot more expensive than it used to be. You always need at least one main body and one backup at around £3500 each (yes, your D90 is a lovely camera but it isn't weather-sealed, doesn't write to two cards simultaneously, doesn't take full-frame lenses, etc), so two bodies replaced every two years is £3500/year. Add on lens replacement (less frequent), flashguns (fairly frequent replacement), bags, etc, etc, and you won't get much under £6k/year.
Ditto computer kit (high-end PC needed for speedy processing, high-end monitor plus calibration kit for exposure and colour accuracy, high-end printer for volume), say £2k/year.
Event photography often means working early or late or both, and carrying a lot of kit, which means you need a
reliable car. Depreciation and running costs say £4k/year.
Insurances (public liability, kit, etc), say £1k year.
Miscellaneous other expenses (advertising, websites, general office stuff, heat & light, etc, etc), say £5k/year.
So before the photographer has even got to the event, that's £38k/year. Let's say they have an event every single weekend, that's 50 events. So before they even press the shutter, they need to make £38k in 50 events, which means they need £760 from each event just to break even.
Let's say there are 200 competitors in an event, and 20% will buy a photo. They need to make £19 from each of those competitors just to break even. That's after they've paid the 5p for the print and quid for the envelope and postage.
So no, a print doesn't cost 5p when you want it taken by a full-time pro photographer. And sure, an amateur or weekender can do it for less, because they don't need it to pay their mortgage and gas bill.
(Yes, I'm a photographer. No, I don't do event photography because it's too poorly paid.)