For pistol brass, I never buy fewer than 1000 in .44-45. Brass is usually shipped in the USPS flat rate box for just under $10. There is no way I'd buy used brass and have shipping be more than 10% of the total in shipping.
In the smaller calibers, it's worse. Since every Law Enforcement pro on the planet (so it seems) shoots .40, there are TONS of it on the market, so the prices are way down. The last time I bought some .45 once-fired brass online, the seller threw in (for FREE) 1,000 .40 empties as a bonus.
When you get to 9mm, the brass is so useless I don't even pay a penny apiece anymore. I don't even bother to pick up my 9mm brass for reloading anymore, because I can get once fired brass from the local police range simply for picking it up.