Combi-Fares

Is anyone familiar with combi fares?
Paid £377 for 3 reward flights (2A 1C) in PE from CUN-LGW, and trying to get an idea of what i'd be looking at for a cash fare LGW-CUN again 2 adults 1 child, probably going out in EC (would it be possible to upgrade using miles to PE? Guess its dependent on the code?)
How is it calculated?
Paid £377 for 3 reward flights (2A 1C) in PE from CUN-LGW, and trying to get an idea of what i'd be looking at for a cash fare LGW-CUN again 2 adults 1 child, probably going out in EC (would it be possible to upgrade using miles to PE? Guess its dependent on the code?)
How is it calculated?