Use longer odds
The refund only matters if you lose. A bet at +200 loses about two thirds of the time, so you are likely to collect the refund, and you win big when it hits. A heavy favorite almost never uses the insurance.
What it is worth
Treat the refund as about 60 to 70% of its face value, because that is what a bonus bet converts to. The calculator below adds it up for any price.