For example, if the outcome of the election has Candidate X ahead of Y by D votes (D = X - Y) and there are less than D provisional ballots to count, is there a reason to count them?
(This being for a simple one-post election; if there are multiple posts where proportion of votes allows for a seat being won/lost by a party, the formula would have to be different.)
Highly variable, but I'm pretty sure it's a lot more to do with the laws of the states than with whether there are enough provisional votes to matter. In looking up some things for an earlier comment I saw one mention of persons casting a provisional ballot then having 3 days to bring proof of eligibility to the county Board of Elections and a separate mention (for another state) of the Board of Elections investigating provisional ballots and informing voters if their ballot was discarded instead.
I'd be surprised if anyplace had a law of "you don't have to count them if there aren't enough to change the outcome," but I've been surprised before.