1. You can’t build it because of America’s artificially inflated profit margin models and costing. Materials for an RV7 or a 172 will be far cheaper in China, India, name your place. Only in America can you demand $300-$400k for a medical school education, and GA isn’t cheap.
Materials cost the same *everywhere*. That's the thing about a global market.
As far as an artificially inflated profit margin, that must explain how Mooney went under, Beechcraft got absorbed, Aeronca got turned into a component manufacturer, Stinson went away, Grumman's Tiger line disappeared, Funk, Globe, Ryan, and a dozen others have just disappeared over the years. With few exceptions, the problem in aviation is too little profit, not too much.
Back to the RV-7. The 180 hp engine is $31K. The prop (fixed pitch) is $4K, and the firewall forward kit (you do want an exhaust, oil cooler, baffles, hoses,etc, right?) is over $4K. So you're at $40K FWF. The kit itself is $28K but that doesn't include a few add-on bits like an airbox for your fuel injection or carb, seatbelts, interior, strobes, landing lights, battery, alternator, voltage regulator, and an ELT. Better add $10K for all that. Now you're at $78K and don't have paint or a panel or all that other miscellaneous stuff that drains your bank account. If you're like everyone else, you're gonna put a two screen Garmin setup in the airplane, with IFR navigator, two radios, ADSB, and an autopilot. You're pushing $40K for the panel. I assume your new airplane will have a coat of paint, right? $10K for prep and a coat of white paint, plus a few vinyl stripes. Now you've got a $125K airplane. Maybe Cessna can save a few bucks with vertical integration, but they are also buying 500 pounds more airplane stuff than you have to buy for an RV-7, just because the 172 is a substantially bigger airframe. More aluminum, more steel. A heavy, steerable nose gear, for instance. A second set of seats and a second set of seatbelts. I'd be surprised if Cessna can "kit" the pieces for a 172 for much less than $125K. For reference, a bare bones RV-10, which is the same size airplane as a C-172 will cost you $150K in pieces and parts. Back out a few bucks to substitute the 4 cylinder/fixed pitch setup for the -10's 6 cylinder/constant speed prop and you're right back in the $125K ballpark.
Regarding the cheap 172, if you want to offshore the thing to somewhere where the loaded labor rate is $20/hr (maybe Vietnam), not $100+/hr like in the US, you'll save money on labor, but you're as far away as possible from the source of everything you need like rivets, tires, wheels, instrumentation, bolts, etc. To run it efficiently, you'll need a mountain of inventory or you'll be FEDEX's best customer, constantly buying batches of various sizes of everything you see in the Aircraft Spruce catalog. Fedexing stuff around the world ain't cheap. And with customs, it ain't fast either. Being a world away from your suppliers isn't a fun way to run a business. Been there, done that. Wait. You're gonna save money and make your own AN hardware? Add another few hundred million in infrastructure and overhead to <essentially> duplicate an industrial supply chain. Oh yeah, once your Vietnamese 172 is complete, you have to ship it to the US for final assembly, certification, test flying, and distribution. Add a minimum of $10K per airframe plus all the inventory of unfinished and finished aircraft you'll have sitting around from time to time, given the ebbs and flows of the economy.
No doubt, it costs Cessna $200K to roll a C-172 out the door, without profit or liability coverage. That Vietnamese C-172 probably costs you almost as much, delivered to the US, and it requires you to invest a hundred million dollars or more into a factory and tooling in Vietnam. The Vietnam option doesn't sound so good. The thing Cessna currently has going for it is that all of the tooling and buildings they use for the 172 line are fully depreciated. There is little new investment required to keep on keeping on. Any new competitor is gonna have to invest in a lot of dedicated tooling and fixtures. Then recover the investment by selling airplanes.
Can all of this be helped by economies of scale? Yes. Should a G650 cost $8K? No way. If you guaranteed to buy 1000/year for the next 10 years, I bet you could get a deal. Engines and props too. But with all of that, we're still talking saving $10-20-30K. Not $100k.
If you want a $100K C-172 equivalent, I bet you could do it IF you sold 50,000 a year. Invest in automation and tooling like Detroit does, and you could get there. The question is... Who's gonna buy 'em, and will they buy at that volume for long enough to pay back your investment?
No.