The main advantage of tying it to time is that you can let people plan for it. If you have dynamic pricing based on unpredictable conditions (how many people are in the cafeteria right now?) you can't plan for it, and the best case scenario is that employees get up, go to the cafeteria, see that the demand/price is too high at the moment, go back to their desk, come back 10 minutes later, repeat until the price/demand is low enough.

pub/sub the queue time discount automatically from register transaction rate; client side compile local vendors and approximate arrival rate by gps.