No doubt it varies between GBBR manufacturers...
If you start high end and add high end parts to get it working reliably...toss any "typical" price total out the window. But that's the same with anything.
If you start mid-low end (est $300-400 for the rifle)...and add $300-500 worth of parts to get it skirmishable...you're at ~$600-900 for the working rifle. Then add 9 mags at approx $30-60/mag ($270-540) and you're at a total of $870-1450.
~$850-1400 isn't too far off the "typical" range I tried to describe, is it?
The ONLY issue I have with that last example though is...you (figurative) may be starting off with a base rifle that is "too low end". I've found that there's a lower floor level where it doesn't matter how much time/money you put into a really cheaply made gun...it just won't be at the same quality/reliability/performance level at the end as starting with a base that's at a higher quality level and adding stuff.
Just my $0.02...