Also you can earn just as much in the Prem.
Debatable. Whilst there is talk of the likes of Piutau earning megabucks at Bristol, it's unlikely that such figures are sustainable at present. Besides, if Manu were to receive anything like what Racing are supposed to be offering, then Tigers would need to seriously adjust their squad accordingly. That's without factoring the French clubs having a seemingly magical ability to simply conjure up a number that they pay their players, regardless of any financial constraints.
Personally, I think Tigers shouldn't fight too hard to keep him. If he's being offered the amounts of money that would set him up for life (and his family of course), then he'd be wise to accept it - especially with his history of injuries. Similarly, Tigers would be foolish IMO to tie up a big chunk of their salary allowance on a player who has struggled to put together a decent run of games in the past few years.
At the same time, it will be sad if he goes, from the perspective of club loyalty. Tigers have supported him and stood by him through a lot of trails and tribulations.