Frankly, Beech is going to make enough money on King Airs for the ISR mission.
What they are promising for this mission is a political fight today, to meet yesterday's requirements, the day after tomorrow.
On the other hand, these are cheap aircraft and we need them now. An initial procurement of the Tucano for near term needs with with a follow on buy of the the Beech makes sense but that is not how our military works.
Both are suited to low intensity bush war COIN type missions and would not last long in any serious conflict against a modern, moderately well equipped opponent. Aircraft of this type just got creamed by the Brits in the Falklands decades ago with 1960’s vintage weapons and were out of the fight in the first few days.
In reality, a modernized, stripped down, lightened A-10 with more fuel efficient engines and optimized for ground attack with precision munitions instead of anti tank operations would make a lot of sense.
It would be more survivable and effective in a future, higher intensity conflict. It would have vastly higher payload of weapons and would not cost that much more than either the Beech or the Tucano.
In reality, most of the costs in military aircraft these days are in avionics black boxes - not the airframe or engine. It is also a fact that the A-10 was designed from the git go as an attack plane and the other two are converted training aircraft.