In a situation like that, where there is a presumption of fault on both sides, fault would probably end up being split 50-50. Which in practice usually means each party ends up paying their own costs. A large disparity in costs (as would probably be the case in the example you give) might change that, but it would probably end up having to be decided in court.
(Note that what I am describing would be the case under current law wherever jaywalking is illegal, not just under the hypothetical law I described.)
(Note that what I am describing would be the case under current law wherever jaywalking is illegal, not just under the hypothetical law I described.)