There is no income tax in Florida, which helps. New York's top bracket is something like 9%, if I recall correctly (something I only know because of onilne hockey chat hfboards.com - hahaha). That makes some amount of difference over the life of a 7yr contract, I'd think. Plus Tampa is the only team that can offer him an 8yr deal.
Neat little tool here: http://gavingroup.ca/personal-income-tax-rates-in-nhl-cities/
$8.5M in Tampa (FL) pays a tax of about $3.3, so Stamkos keeps $5.2
$12M in Buffalo (NY) pays a tax of about $5.7, so Stamkos keeps $6.3
$10M in Detroit (MI) pays a tax of about $4.5, so Stamkos keeps $5.5
$10.5M in Toronto (ON) pays a tax of about $5.6, so Stamkos keeps $4.9
I used different contract values for each of the likely landing spots. These are the rumored numbers that each club may offer.