Not just cheap, but free if you're a wine drinker, or know one. Wine boxes! Yup, those rectangular 5-liter "drink it by the gallon" boxes o' booze.
Here's how.
1. Drink the wine & sober up.
2. Pop out the little black "faucet", by separating it from the whitish plastic lip to the internal "baggies".
3. Rinse out the bladder/baggies with tap water to prevent a sour smell later.
4. set up the bladder with the opening at the top & fill completely with tap water.
5. re-insert the faucet by pushing it back into the friction fit collar.
6. Stack as many as you need back to back to trap the fired bullet. (6~7 seems about right for a .357 mag pistol). You can tape tie or whatever them together to get a good tight fit.
Fire into the center & recover the bullet by looking at how many are perforated.
Before & after of a Hornady 125 Gr XTP @ 1350 FPS fired from 15 yds done this way.