What I would do is turn the prices for the full field into percentages in the time honoured way (I'm pretty sure you know how to do that - same way VFG adds the percentages up at the bottom of each race).
Remove the horses that are scratched and redistribute the percentages proportionately between those left. For e.g. if you start with three horses at 50%, 25% and 25% and one of the 25 percenters is scratched you have two left at 50% and 25% which now share the 100% between them so they become 66% and 33%.
Then turn those percents back into the new odds (in the reverse time honoured way).

Apologies for a non detailed work through but short of time and I know you with your level of expertise will cope with this.

KV