dragons den businesses

Dragon Den and businesses?
How do the dragons value the businesses proposals before them despite having none or little trading records to hand?
Because I cannot understand how a percentage share of the business is worth a X amount.
Firstly, they expect the people presenting the ideas to have records of sales etc. they’ve made to date and if they have no data then an extra risk is added and the Dragons will want a bigger percentage. There will be plenty of paperwork behind the scenes.
The Elevator Pitch