We have to advise on the pricing per backorder, but they are similar to restores. You will only be billed if/when the backorder is successful.
A backordering service will attempt to procure the domain for you as soon as it's fully deleted and released by the registry. It is not a guarantee, but does increase the odds of securing the domain. Unfortunately, if you just try to keep an eye on the domain's availability at the deletion stage the odds of getting to it first are much lower.