We use a set of those lifters you linked for our king-size bed. They work great. I would NOT use milk crates, as their open frame isn't designed for point-weight support like a bed frame foot.
If you're looking for a low-cost lift of a couple inches, a small piece of lumber (4" length of 2x4, for example) would work fine. Be aware, most basic bed frames have wheels, and can roll off blocks of wood. This can be a bit shocking, as it's likely to happen when you're moving on/off the bed. One advantage of the $20 lifters is they have a lip which traps the wheel. They also interlock, so if you need to lift the bed a bit higher (2 lifters worth) it's still shear safe.
Whatever you place under the feet of the bed, make sure you raise ALL of the bed feet the same distance. Larger beds (kings, some queens) have supports in the middle of the bed as well as corner supports; they must all be lifted equally. Other than that, anything which will support the weight without collapsing will work.
The above applies to basic angle-brace steel frames. More sophisticated/expensive frames which incorporated head or foot boards or other features may not be liftable this way. Well... the basic idea of "lift ALL the feet equally" should still apply, but you may have to do something to also lift the header/footer. These concerns will be much more frame specific, and we'd need more information (pictures?) to properly answer.