The AVG were paid nothing by the US. They were paid by the Chinese government and awarded medals for their service by the Chinese government. AVG men were paid in US dollars ("gold" in the slang of the AVG as opposed to "Mex", the term for Chinese currency) but the money came from the Chinese, not the US. When AVG went to Africa to ferry P-40E's back to China, their expenses (food, hotels, spending money, etc.) were paid by the Chinese government, not the US. The US did give money to China, but such money was usually in the form of lump sum loans, not itemized for specific things like "funding the AVG" or anything like that. In that sense you could say that US funds were paying the AVG, but only in the same way that US funds were helping to pay for the Chinese war effort as a whole.
There was no doubt that many men joined the AVG for the money and for the money alone. A lot of these dropped out as soon as they realized that they would not be picking off undefended bombers but would instead be facing Japanese fighters at incredible odds. Others joined for the adventure or the opportunity for action. Others, like Boyington, sought to escape stateside problems. Some genuinely wanted to fight for China. Almost all of them joined for a combination of these reasons. Charles Bond, for instance joined because he could not get a regular Army commission and hoped that combat experience would get it for him, especially as he felt that the US and Japan would soon be at war. However, I don't think any of them saw themselves as mercenaries. Even those that joined for the money did so with the understanding that they would rejoin the US military at some point if war broke out (except for those recruited from outside the US military), as most of them did. That being said, the AVG men generally revelled in being a non-regulation unit without rank or standard uniforms (except when they wanted to wear them). I brought up the issue of rank, by the way, only because the AVG men generally prided themselves on being non-military in that fashion. Some, more traditional, military men among them (like Charlie Bond) criticized the lack of Army discipline, but most of them loved the egalitarian spirit of the AVG and felt it helped make the AVG so successful in combat and formed tight bonds and loyalty to each other that exists among AVG vets to this day. The only people generally disliked by AVG vets were those that broke their contracts and left early or who were kicked out for various reasons. This is why Boyington, who, depending on who is telling the story, either quit or was fired, is such a "black sheep" (sorry, I couldn't resist) in the AVG family. Loyalty, to the unit, each other, to the "Old Man", and even to Chiang Kai-shek and the Nationalists, is the most prized quality of many AVG men I have met or talked to, a very unusual quality to be found among men regarded as "mercenaries".