I assume the $290 was the result of the commission on anticipated stadium expenses that the Brewers commissioned last summer, but I couldn't immediately find the results of that. Otherwise, I agree it's oddly specific without some sort of backing.
In the end, the Brewers' lease required the stadium district to pay for maintenance expenses and by many accounts, they lack the funds to do so. Essentially their choice is to break the lease with the Brewers, which they won't do, or acquire additional funding. Since this is reapportionment of already anticipated revenue rather than adding the 5 county back on to raise additional revenue, it seems like the least burdensome of the limited options, knowing that the funds are obligated to come from someplace.