I see people saying a lot of things about signing on fees, wages etc. It's true he might get more as a free transfer, but it's also worth considering that even regular transfers involve player signing on fees.
Very important to remember that many Prem clubs aren't exactly struggling or tight with money, so they're very generous with signing on fees and wages - let's say he went to Villa, Newcastle or Spurs for a £10mill release clause next summer... He'll be made VERY rich by them.
In the mean time we can make him pretty rich in the season he's "seeing out his contract", e.g. payrise to £50k a week for 12 months.
NOTE: I've done no significant maths here, it's just a reminder he will be rich however he moves, bosman or even £40mill full fat transfer club to club.