Yeah, those Firing Line receivers are very nice, but pricey. Since Arsenal Inc does not sell their SA M-7 receivers as stand alone components, the Firing Line would be the only company you can get a stand alone milled receiver from.
It's true about the SA M-7 being a good price for what one gets. Arsenal Inc uses a milled from forgings receiver (as done by Arsenal of Bulgaria), instead of a milled from barstock receiver (as done by the Firing Line).
With all else being equal, the milled from forgings method will produce a better overall product. The problem is the expense that the forging dies and foundry contracting brings when that method is used.
Here is what Queen City Forging has to say when comparing the milled from forgings method to the milled from barstock method:
"Machined bar and plate may be more susceptible to fatigue and stress corrosion because machining cuts material grain pattern. In most cases, forging yields a grain structure oriented to the part shape, resulting in optimum strength, ductility and resistance to impact and fatigue."
As one can see, when everything is equal, the milling from forgings is the better way to go when going with milled, but the process can cost more because of the tooling (forging dies) and outside foundry costs.