Currently, many of the military/experimental aircraft that the devs like making (such as the YH-32 and XF5F) aren't being bought or used by the majority of the playerbase, and understandably so. Why put WP or UP into these vehicles when you'd be better served by an airliner or cargo plane? However, some of these military aircraft (like the SR-71) are great money-makers, often making more money than the best cargo planes and airliners. This is mostly due to surveillance, races, and PTS (the last one less so now that it's been nerfed).
However, Derail Valley recently released an update which added the DM1U - a railbus that's weak at hauling, but which can deliver parts to restore player-owned demonstrator locomotives that have various benefits (such as being able to be spawned anywhere and not having to pay emissions fees).
There could be a couple ways to add this bonus. I've included a table with three formulas (a flat 1% bonus, a flat 3% bonus for museum airports like Yoeshaw if the vehicles are kept there, and a price^0.55 bonus) on vehicles of various prices to show how the bonuses could work. This bonus would only apply to charter and cargo flights.
As for exceptions, I'm thinking that surveillance aircraft could be excluded from giving this bonus (as they make too much money too easily), and SSTs and corporate aircraft could be excluded from receiving this bonus (and possibly helis/VTOLs, as well, as they already get enough bonuses as it is). Boats likely also will have to be excluded (definitely from the flat bonuses, but the ^0.55 bonus might work with them).




