Let’s look into what happens to horses’ prices when one of them is withdrawn from a race.
Since the winning probabilities of other horses will thus increase, their cumulative price (book %) will naturally decrease (otherwise we would be having an overround).
However, for some horses, the prices drop truly dramatically, which is obvious from the price graphs (the dashed lines denote a withdrawn runner).
To be clear, the prices of other horses will therefore soar at the same moment (which makes sense, as they need to compensate for the lowering of other prices). Those horses we can ignore.
What we want to focus on is the horses who see the biggest price drop. Their prices will eventually climb back up, so you will have a good opportunity for a green-up.
https://bfcharts.co.uk/