Gunner stats can be a cheap way to get a few extra levels to get you up to your qualification requirement though so you can get expert in a plane.
When your pilot stats might be 10-25 per level, your gunner stats might still be down in the 2-5 per level range.
Was about to say this you ninja!
Also, does cash reward depend on tiers??
Depends on the plane, which is sort of losely based on tier. In each plane's stat window, when you hover your mouse over them, it should have a percentage which tells you how much of a reward you get relative to other planes. The reserve planes get you next to nothing to start with. But even two different planes of somewhat equal tier won't always get anywhere near the same reward. It's also based on game mode. Arcade gets the least and full real battles get the most, while historical battles get somewhere in between, but usually much closer to the full real battle side.
Example. US tree: Tier 5 TBF gets 100% reward in arcade. Tier 5 A-20 gets 90%. Tier 5 F4F gets 70% Tier 6 Airacobra gets 50%
Notable difference in the German tree: Ju 87 G-1 with the dual 37mm cannons gets 140% reward while the G-2, an identical plane (except it carries a double ammo load for the cannons) only gets 90%. Me-410s with the 50mm cannon also gets an amazing 150% while the other 410s are between 70-100%
This seems to be their primary way of "tweaking" the economy and trying to get the lesser used planes to be more attractive. Premium planes tend to, but don't always have double the reward of their researchable counterparts.
Scale doesn't stop at 100% either, but doesn't seem to go past 150%, except in the cases of premium planes where the base reward is above 75%.