Quick update: Sigma's newly announced 150-600mm f/5-6.3 DG OS HSM Sports Lens (unveiled last week) already has a confirmed price point: $1,999. It will be available some time in October, with a specific date not yet given. At that price point, it's much higher than the Tamron we reviewed earlier this year, so Sigma must believe the performance and build quality to outperform the Tamron.
You can pre-order the Sigma here. No pricing or availability was revealed for the Contemporary version of the lens at this time.