Just remember that anyone to do with avionics is a professional bull****ter of the first order.
Half of it is that every installation is different and the other half is that there are so many variables. And its usually easier to baffle a pilot with BS than it is to dazzel them with brillance. You can charge the same it takes a fraction of the time and ensures repeat buisness.
You will need to go to more than one place to get a quote and also get more than one opinion.
I would imagine A and C has pretty much seen every variation you can get in a Cessna 150/152.