I am amazed at the price they get for the interlock device. It is a stamped out piece of sheet metal and some mounting hardware that costs over $100 when I bought mine. All it really does is make sure you can't have the main breaker on when you turn on the back feed breaker from the generator. No other switching is involved. I assume the price reflects what it costs to get it listed and the lawyer tax for potential liability.
The 30a (L14-30x) devices are at Home Depot and Lowes along with pre made cords so that part is easy and they do sell the 6 breaker transfer switch but I doubt many homeowners will screw with that.
The reality is my 5.5kw Briggs generator will run my general lighting load if I turn off the central air, water heater and don't use the range. I don't have a separate generator panel. In the 20 years I have had this generator, I used it 3 times. Our power is generally very good here and it is only after a storm like Ian or Irma that it is out long enough to worry about a generator. I also had one PoCo maintenance related outage that lasted several hours.
I do know most of my neighbors do the dryer plug thing. They find out pretty fast that if they forget to trip the main, the generator trips out when it hits the locked rotor of their neighbor's HVAC.
The smart ones have all the breakers they need to trip marked with nail polish or something. I cringe a little when I see it but I ain't a cop.
