from explanation-guide.info

A supermodel is a famous and extremely highly paid fashion model. The term also implies steady access to work, uncommon in the highly volatile fashion industry. They are, almost by definition, sex symbols. The term emerged in the 1970s, though a number of models had become famous in their own right as far back as Dorian Leigh in the late 1940s.

Supermodels of today are globally famous, and parlay their celebrity for product endorsement deals and often into acting careers. Supermodels who have made the switch include Milla Jovovich, Elle Macpherson and Rebecca Romijn-Stamos.

So the prime factor seems to be fame rather than money.