I have a hard time believing Total can offer 5.5 million to any rider, let alone want to do so to a washed-up Sagan. Similar for a broke Astana and an ancient Nibali.
I'm not saying this list is accurate but what some of you seem to be missing is that a rider's salary is not determined by their current sporting level but by their marketing worth. Riders like Froome, Sagan and Nibali are exactly those who you would expect to be overpaid.
First, contracts are not negotiated now but in the past, so many of those riders were closer to their best days when they signed the contracts than they are now, the prospect of them not declining as much by now as they have done must have been considerd.
But the more deciding factor is that sponsors want return and you get return not only by getting good results but also by being talked about. Recognisable riders, even if past their prime, are still popular among fans and pundits and get you more attention, they are better at advertising things etc. That's why their negotiating position is much, much stronger than that of riders of similar current level but without the history of being a top rider in the past. This pattern is very evident in all sports. Your salary is always more dependent on your past achievements than your current level.
For smaller teams, there's another factor at play- they often can't attract top riders in their prime so they go for riders past their prime to get their big, recognisable names and increase brand recognition. In order to make them choose their team instead of a bigger team, they often have to offer a bigger salary than a big team would, thus overpay them. This may be not ideal if all you care about is building the strongest team possible but for sponsors, there's more to sponsoring than that.
Total definetely have some € to spare, they have hunted for a big name for a good few years and made offers to Alaphilippe and Kristoff in the past.