$400 for an intake is actually about the norm.
They're never cheap when the competition is all that price.
These intakes aren't hard to make at all, ESPECIALLY now that the 5.0 has a circular throttle body opening. I'll be making mine whenever I snag a 5.0.
It's the MAF housing that usually is the critical piece. If you can get the maf housing just right, that's the key.
In this article
C&L Air Intake System for the 2011 Mustang
It says how the maf apparently operates differently on the 5.0's so it doesn't require a tune.
check it
(just guessing on the diameter of the intake hose.. it could be 3.5 or 4".. whichever, there are those sizes as well.
A few of these
cxracing.com: 3" Straight Silicon Hose, 3" Length, 3 layer
This
cxracing.com: 3" L-Bend Aluminum Pipe, Mandrel Bent Polished, 2.0mm Thick Tube, 18" Length
And this (which out flows
K&N, Greddy, HKS and every other brand and DOESNT use oil or need re-oiling)
Amazon.com: Fujita F5-300S Super Flow High Performance Air Filter: Automotive
And you've got everything needed for an intake for what... $80~$90 with shipping for the various parts..
All you would have to do is possibly add something like this (this just used as an example, could be a bigger size) if there's a vacuum line or two attached to the intake pipe
Amazon.com: BARB-5/16X1/4-BOX/10: Automotive
I saved almost $1000 building my own full length intake and turbo intercooler piping myself. Works like a champ
I think the biggest question though is going to be if the stock MAF is actually PART of the air box or if it's a small section that can be removed and re-used on the cold air kit or not. If it's part of the air box, I'd try to swing a deal with MMR to see if they'd sell you just the MAF housing for like $100 or so and then use your own piping/filter. You'd still come out WAY ahead.
Also worth noting is that there may be room below the factory air box to add another 90* pipe leaving the MAF (INLET) pointing the filter down around the bumper for cooler air.