Where did you hear it was cheaper?Did you know that Packard had to pay royalties to rollsroyce of $5000.00 US per merlin built under licence?That was a boatload of money that did not stay here in America whereas Pratt&Whitney was an American company that supplied the engines for the P-47s as well as the Corsair,P-61,Hellcat,B-26 and on and on and on. Maintainence was also better on the thunderbolt than the mustang.Just ask any mechanic that works on them today.
Depending on the source the Mustang was about $10K to $30K less or 20 to 60%, which makes sense as most machinery is almost a linear cost function to weight. One of great selling points of the Mustang was ease of manufacture and cost. Hey it 2 less guns, right?
The Thunderbolt would have been clearly a better choice, but I think that the politics may have involved money as well as other factors.