Not knowing TB's finances but I'd say it would have to be a pretty hefty offer.
Given his background I would say the £150m he has invested is probably less than 10% of his worth. I can't seem him risking anymore.
If someone say offered him £200m is that enough. What is £50m to him...
One word - academy setup.
Southampton must be (one of) the best clubs in the county at producing young talent. Imagine what impact this has financially. They have saved millions on player in (by "growing" their own) and made a huge amount from sales.
Off the top of my head -
Bale
Walcott...