Of course the GBBR is still going to fail far more frequently than a comparably priced upgraded AEG. Especially if you include mag prices in the equation.
If a manufacturer machined the major parts like the upper/lower receiver, and trigger mechanisms (minus the trigger box, since that itself doesn't really encounter much stress) out of steel (for high stress/wear parts)/aluminum (for low wear parts), the failure rate would be almost nil. Good rubber for the o-rings would reduce the problem of leaky mags.
The cost of the mags isn't unreasonable, considering what it takes to make them, and put them together.
Part of the cost is demand. There isn't enough demand right now to bring the costs down enough for the manufacturers to make a living, so prices have to be higher.