The purchased pizza has to be at the regular menu price while the second pizza (as usual) must be of equal or lesser value.
As a reference, here's a list of regular menu prices from my local Domino's (yours may vary):
- A Medium, 1-Topping Handmade Pan Pizza is regularly $10.99.
- A Large, 1-Topping Hand-Tossed or Brooklyn Pizza is regularly $11.99.
- A Large American Legends or Feast Pizza is regularly $17.99.
Photo via Domino's.