The easiest way to look at this sort of market is to look at the Asian handicap line and convert across. The total will of course make a difference but this can give you a pretty good idea.
A game where the teams are evenly matched might be bet like this (to 100%):
Team A 3.00
Tie 3.00
Team B 3.00
On the Asian handicap they will bet this as 0 : 0 (ie level ball) 2.00 2.00 (to 100% again).
By giving one team a 1/2 goal start on the Asian handicap, you are essentially taking the tie out of the equation. So the game will be bet as 0 : 1/2 2.00 2.00 (to 100% again).
So, taking this back to the fixed odds, you have:
Team A (to win or draw) 2.00
Team B (to win) 2.00
So, essentially, you are going from a 33% chance of winning to a 50% chance of winning for the change from being evenly matched to having a 1/2 goal difference.
I think I've got that right. It's a bit early in the morning and I'm trying to watch Ukraine at the same time too.