The left handed build portion is what is making it so expense.  The left handed build market is probably much less than 1% of AR15 sales.  If you want to find a cheaper way to build a weapon, make it a standard AR15 and you should save a chunk.

I am not sure if you are looking for a complete upper receiver group or just a upper receiver assembly?

A complete upper receiver group would be the upper receiver assembly plus barrel, etc.
The upper receiver assembly is just the upper receiver with ejection cover and f. assist installed.

In any event, you will need both the upper receiver group and the bolt carrier group.
Both are built as a mirror image of the right hand, so the notches on the carrier for the forward assist are on the left and the bolt rotates the reverse direction.

I would contact Stag just to comfirm the interchangablity of parts with your 11.5" barrel.
(just to be sure)