I know a solution.
Let's modify the cost of holding the timers by the percentage of the players playing at specific hour, compared to peak hours.
For example, each week, admin takes a look at active player counts.
Then, we divide the regular cost, by percentage of players to have final cost.
For visualization I'll use "100" gold as a base.
1) PB Timer set for peak hours (600 players in average)
100 gold : 100% = still 100 gold
2) PB Timer set for off-peak hours (300 players on average)
100 gold : 50% = 200 gold
3) PB Timer set for time when lowest number players is online (60)
100 gold : 10% = 1000 gold