VM sells the Motorola Triumph for $300 so another $150 isn't an outrageous price.
That is a 50% increase in price. Consumers in the $200-300 budget price range tend not to be that price elastic. If they were so price elastic why are they on Virgin Mobile shaving dollars off their monthly cell bill ?
The Triumph is a huge stretch for most Virgin customers. At $450 that would be would be an even bigger stretch.
Adding an iPhone would only be adding more of a status symbol product to the line up than actually getting significant volume in sales ( plus trade off the short/intermediate term negative impact on parent Sprint and it is non-starter. )