This seems silly. Why should corsairs be singled out for their choice of ammunition when it comes to buying more of it?
Let's compare rangers and corsairs as one of many examples. Rangers only need weapons, ammunition, and equipment. Like any other physical damage-dealing job, a ranger will wear equipment that helps them gain TP faster while also making their shots both more powerful and more accurate. When using a weapon skill, rangers will switch to equipment that adds to the main attributes the chosen weapon skill draws from for its damage. Just about the only two reasons a ranger would even need to use an axe or a dagger are if they're out of ammunition (arrows or bolts, because rangers don't use typically use guns) or if they gained too much enmity.
And that's just a party situation.
A corsair has to buy similar equipment, weapons, and ammunition. But in addition to that, corsairs have to buy the dice needed to learn Phantom Rolls, and bullets for guns are much harder to come by compared to arrows and bolts for bows (which corsairs can't use) and crossbows (which are very uncommon for corsairs).
Bullets are more expensive than arrows or bolts because of their scarcity. Making bullets a reward on the Walk of Echoes or something you get in exchange for specific content points is ridiculous when every merchant NPC and their mothers sell arrows and bolts. This imbalance lets anyone who takes up smithing make a fortune from just bullets, even compared to arrows and bolts.
There needs to be a balance between all of this, and making bullets a special reward of any kind won't do that. You need to have more merchant NPCs sell bullets, or you need to have less merchant NPCs sell arrows/bolts. It's as simple as that.