Yes it would be enough money that he wouldn't have to work again, but so would another PL contract. As he is a free agent, he could easily sign a 4 year deal on £50k+, that's £10m! I know that'll be taxed but you also get all the bonuses and signing on fees. Not to mention he has just had 4 years at Saints on over £30k/week too.
I know its hard to say until you are in that position, but if I was already a multi-millionaire then I'd rather the £10m contract and another 4 or 5 years at the top level in England and not disrupt my young family than the £20m contract and have to either live away for years or move your family out to Saudi.