You can buy cheap flint stones from the ah, cheap copper ores from a npc and make your own minnows for about the cost of the minnows sold at the guild, roughly 2k.
Not sure why you are buying stacks of brass for 40k, stack of copper is about 6k, 4k of that being the stack of fire crystals.
Probably comes pretty close to profit and value compared to sardine balls, fewer synths too.