Seeing as Millwall are paying the entirety of his wages and their average player wage is reported to be around £7k a week (one of the lowest in the league), I would think it's highly unlikely that he's on £16k a week. When Bogle got done for speeding, it was reported that he's on £9k a week, so I'd be amazed if Bennett's somehow got himself on £7k more than that. To be honest, Bennett's probably on much less than Millwall's average, even. The deal he signed running to the end of this year was signed in 2017 when he'd only played 786 minutes of senior football for us across 6 years and so was barely any more than a youth player, and with his injury record and lack of game time this season, I also just don't think Millwall would bring him in as a high earner, especially on full wages. It actually wouldn't surprise me if he was one of the players mentioned as being on less than £6k a week when deferrals were being discussed not long ago.
To me, extending his contract by a year would therefore make sense, seeing as it would generally have been a fairly low-risk move. For a start, Millwall reportedly have a £500,000 option to buy at the end of the loan, so if all went smoothly, we pay none of his wages from January to the end of the season, we make a profit on an academy graduate, and his contract next season doesn't begin. Even if Millwall decline to do this, we could still have the opportunity to make a deal, any deal, to sell him to another club either at this level or lower, with the exact same effect as selling him to Millwall.
If that fails, we loan him for the year with the loaning team paying his wages until his contract ran out. I know a lot of people here really don't rate him, and I'm not going to get dragged into arguing about that, but as an objective fact last season he recorded 3 goals and 4 assists, 7 goal contributions total. He's demonstrated that he can put up those numbers in a season at this level. At many clubs in this league, that would be a good record: only one player at Wigan has more than seven this season, only three at Millwall, only two at Luton, etc. I don't think it would be unlikely that a lower Championship team or League One team would look at that record and be willing to take a risk, even if it was just to get another body in the team, especially if they know they can get him for free if he does do well. If this were the case we see him walk away for free at no more cost than letting him walk at the end of this year.
Absolute worst case scenario is we are unable to get him out on loan, can't reintegrate him in the squad, and he just plays in the reserves until the end of the season. Considering that his wages are almost certainly one of the lowest in the squad, he would end up as no more than a low-cost Anya substitute. Even if he were on £7k a week (he's probably not even on that), that would only cost us £364k over a year, which would be fairly unsubstantial when our total wage bill was last reported to be around £46.8m. This could be worked around if necessary.
So, for me, it was a good move at the time to do it. Of course, Coronavirus has been an entirely unforeseen event that's thrown everything into disarray, and I have no idea how things will pan out now, but really, Bennett's contract will be nowhere near the biggest of Derby's worries when this is all over.
As a side note, Martin, I think, is on £30k a week, or at least that was what was reported when he was on loan at Fulham.