Here is a spreadsheet I created that attempts to recreate actual dollars expended on 40-man payroll in 2015 by applying their per-day salaries to the actual numbers of days they're on the roster. (This way, for instance, Ben Zobrist's presence on the 25-man on Opening Day doesn't artificially skew the figure upward, and his absence on the end-of-year roster doesn't artificially skew it downward.)

The total I came up with, which includes the money they owed Nick Punto and the money sent to Kansas City and New York in the Ben Zobrist and Tyler Clippard trades, but does not include any cash coming back when the amount wasn't reported (e.g. in the Edward Mujica deal), is a bit over $82 million. Cot's Contracts calculated the Opening Day 25-man roster as a bit under $84 million, and the end-of-season 40-man roster at a bit above $84 million, for context. I was surprised to see the figure come in so close to those estimates, but I expect that if you did this exercise for all 30 teams, you'd find more variation, some teams paying much more than either the Opening Day or end-of-year amount and some teams paying much less.

I had to make some assumptions, fill in some values, and generally deal with a lack of information in certain ways:

• For any player for whom I could not find a major-league salary, I assigned a salary $507,500, which was the major-league minimum for 2015.
• To estimate the minor-league minimum salary for a 40-man-roster player, I found the cost-of-living increase for 2015 over 2014 and multiplied that by 2014's minor-league minimums. Therefore, for any player who had, prior to 2015, appeared in the major leagues, I assigned a minor-league salary of $82,725. For anyone who had not made such an appearance, I assigned a salary of $41,365. (The 2014 figures I used for this are in the CBA.)
• A more meaningful salary figure, even ignoring the pittance paid to most minor-league players, would incorporate salaries for minor-league free agents like Andy Parrino and Pat Venditte. I would guess that these players are not making the minimum salary because they were able to sign with the highest (minor-league) bidder and thus were able to negotiate something above the minimum, whether on or off the 40-man roster. Those salaries are almost never reported, though. Therefore, when Parrino was outrighted off the 40-man, I just stopped his line item. Similarly, for the period when Venditte was in the minors before the A's selected his contract, he doesn't count for anything against the total.
• On the other hand, for players outside the minimum-salary years, I assume they're not signing split contracts and are thus being paid their full major-league salary even when in the minors—see Craig Gentry, for instance.

Feel free to muddle about with the spreadsheet and tell me what I've done wrong.