Neither brand revealed pricing or release dates for the iPhone 6s. In the past Boost and Virgin iPhones, which are locked until they have been used on Boost or Virgin for 12 months, have been priced $100 below what Apple charges for the equivalent unlocked version. Apple is asking $649 for the 16 GB iPhone 6s, $749 for the 64 GB model and $849 for the 128 GB version, so I expect Boost and Virgin iPhone prices to be $549, $649 and $749 for the 16, 64 and 128 GB models respectively.
As to when the iPhone 6s will be available from Boost and Virgin, that's anybody's guess. Last year Boost got the iPhone 6 29 days after it went on sale at Apple Stores. The iPhone 6s will be in Apple Stores starting Sept. 25 so I'm guessing it should be available on Boost before the end of October.
With Virgin, the big question is if the iPhone 6s will actually be in stock on the Virgin Mobile site. Virgin stopped selling iPhones late in 2014. Virgin Mobile iPhone sales resumed in July of this year, but the iPhones quickly went out of stock and are still out of stock on the Virgin site. It's also almost impossible to find a Virgin Mobile iPhone 6 at the big box stores that sell other Virgin phones such as Walmart, Target and Best Buy. Actually the supply of all Virgin Mobile phones seems to be drying up, leading many to speculate that Sprint is going to discontinue the brand. Hopefully for Virgin Mobile customers, the iPhone 6s will mark the beginning of the revitalization of the Virgin Mobile brand in the US.
Prepaid Operator Profile: Boost Mobile
Prepaid Operator Profile: Virgin Mobile
The iPhone Returns to Virgin Mobile - 5c, 6 and 6 Plus Now Available