There are a few other things you can do if you want to beef them up a little more.
1) Prying usually functions by bending the out-side frame and the door appart. You can reinforce these ares with some common angle iron and or square tubing. Measure the inside depth of the door (they're normally recessed) and pick up piece of stock that just fits. Cut the fill width and brace the door at top, bottom and either side of the lock (takes 4 bars) - If they use a veritcal bar lock, you'll need to cut the bar stock for the vertical locks. Then either add a full length piece to the open-side of the door frame to prevent it from bending or 4 short reinforcing bars horizontally from the inside of the door frame to the outer wall of the cabinate.
2) The locking mechanisms generally pass through fairly light metal and are typically somewhat weak. You can reinforce the cabinate frame that the locking bars pass through with a small piece of flat stock (target 1/8") with a cut-out for the locking bar to pass through. You can also replace the locking bars with stronger / thicker metal. You only need to reinforce the bard an inch or two into the door area, so you don't need to replace the entire locking bar on a vertical top/bottom locking system, just the last 3-5 inches.
3) When you mount it to the wall, select your bolt size (I'm a huge fan of 3" long 3/8" Tap-Con Stainless Steel Masonry Bolts) then pick out the largest fender washer that will fit that bolt. For even stronger mounting, pick up 2 pieces of 2" wide 1/8" thick steel bar stock and drill 2 holes that just fit your bolts the same spacing as the cabinate holes and use that as your "washer"
Yes, there's a lot of work described there and it's still not as secure as a proper safe, but for materials, you're adding $10-$20 to a $100 cabinate rather than buying a $500 safe and and you're ending up with something that will require a LOT of work to access.
If you're still worried about someone cutting through the side, reinforce the sides as described for reinforcing the door. 3 bars as the 1/4, 3/4 and 1/2 points. this will force someone cutting through to cut through the bar-stock as well as the thin wall.
For mounting, Rivets or the smooth headed cartiage bolts (have a square piece under the head to lock into a square hole) with the head on the out-side. With thin metal like the cabinates, torquing the bolt will draw deform the metal around the square hole to hold the bolt from turning while preventing it from being turned externally.
