Nice thinking but you will find that avionics installing companies aren't making much on the actual units. In our case we encourage the customers to pay direct so we don't have to tie up cash.
20k - 30k seems middle of the road. Priced a King air upgrade with the UNS about 60k and Collin TDR90's about 25k each.
One suggestion is a Blind GPS like Free Flights 1201 to save on GPS upgrades.
All depends on what hardware you are upgrading. Good luck finding 25 aircraft with similar hardware requirements.
We are installing and near capacity at doing that. Don't need extra work thanks
Even at current install rates I believe the Australian fleet will fall short of the deadline. Get in early people! Remember trying to buy a 406mhz ELT install kit just before the deadline?