In this diary by Cieren, Why Prosser needed EXACTLY - +7500 - votes, it is claimed that:
So they couldn't just give Prosser a couple hundred to put him barely in the lead by 40. They had to give him more than 7500 in order to get above the threshold for a free recount, so that the only way for a recount to happen would be if Kloppenburg ponies up the money for a state-wide recount, which could be expensive.
But I don't think it would be as expensive as folks believe. According to the posted Recount Manual (PDF) the cost, if it was over 7,500 and under approximately 30,000 vote differential, would only be $5 per ward or municipality requested for a recount (Wis. Stat. § 9.01(1)(ag)5):
1m. If the difference between the votes cast for the leading candidate and those cast for the petitioner or the difference between the affirmative and negative votes cast upon any referendum question is at least 10 if 1,000 or less votes are cast or is more than 0.5% but not more than 2% if more than 1,000 votes are cast prior to issuance of any amended return under s. 6.221 (6) (b), the petitioner shall pay a fee of $5 for each ward for which the petition requests a ballot recount, or $5 for each municipality for which the petition requests a recount where no wards exist.
I have no idea how many wards or cities there are, but if there are 1,000 that is still just $5,000, which to me is much less than what would be the real cost.