The nice thing about spending money in support of a candidate is that if that candidate withdraws and chooses to back another candidate then most of that campaign money spent is still a net positive.
Whereas, the GOP's entire attack strategy almost certainly focused on just Biden and not a whole lot of policy attacks. With Biden out, that money may as well be a pile of cash they lit on fire. They've also spent quite a bit on content they haven't released yet that is no longer relevant to this election.
They're pretty good at wasting money if you look at the last few republican presidencies. Spend spend spend, but not on American citizens. That would be socialism.