It depends on the type and your local market.
I have 3 gun clubs within 15 minutes of my house and 12 gauge target hulls can usually be scavenged for free. A lot of the affluent skeet and trap shooters only shoot factory shells.
The most desirable hulls are the old Winchester compression formed AA’s. Those are hard to find once formed and bring a premium.
Next up are extruded Winchester AA’s (separate inner base wad) and Remington STS hulls. Once fired should bring about 4 cents/ea. for 12 gauge, 5-6 cents for 20 gauge and 10 cents or more for 28/.410. The old compression formed AA’s will bring more.
The extruded hulls don’t split at the mouth as quickly, but the inner base wad can come loose and lodge in the barrel. It’s easy to spot in a fixed breech gun (SxS, O/U or SBT), if you’re in the habit of looking down the tubes before dropping in live shells. It’s almost impossible to notice in an autoloaders or pump. You can kiss your barrel goodbye if you fire the gun with a base wad stuck in it.
I’ve never shipped hulls, because of the volume. I typically sell excess at local swap meets. .410/28 gauge hulls may be worth shipping in todays market.