It's 1xp on the whole warband, it's kinda pointless. Winning needs to actually mean something, else it's always best to run away as soon as you go below 50% wounds.Winning will give you that extra boost in EXP.
Different scenarios will provide you with bonus EXP for completing the mission objectives, so keep an eye out for different objectives and try and go for them for the extra EXP at the end.
I'm fairly sure it's just a random number.One thing I would like to see is an actual ingame table showing how much wyrdstone you can get. There are some objectives ingame but I dont know if they have any effect on reward.
Edit: Each enemy taken out of action would be worth 1 shard. Completing objectives each reward shards. Holding a circle for 10 turns for example could be worth 5 shards. Moving to enemy base worth 2 shards per team member.