Earlier this month Ruger announced that they would be selling complete upper receivers for Ruger SR-556. The price difference between a full SR-556 rifle and the upper is just over $500.
The Ruger SR-556 comes with an unexciting lower receiver with a standard M4-style stock. The $500 savings could be applied to a more interesting receiver.
The upper is available in both 5.56mm and 6.8mm. They both have an MSRP $1449.