Usually, cheap and quick don't go together. However, with a minimal effort, you can create really nice looking tags for your plants.
First, you'll need to get a letter and number stamp punch set. These punches are made of hardened chrome vanadium steel and are quite strong and can also be used to punch metals. You'll also need a hammer, but I'll leave that out as it's something that you most likely have at home.
Next, get a set of blank dog tags like those used by the military. Here's 50 of them for $20. That's pretty much all you need. Get punchin!
I'd say for a 1 time investment of $20 (punch set) and $20 (possibly recurring) for the dog tags, you've got yourself a really nice plant name tags that cost less than $1 each (less than 50¢ each for additional tags).
If you don't like these and/or if you want to make it even cheaper, you can cut up beer cans into little rectangular pieces and use the punch on them. In this case, make sure that you have a firm wooden backing for the pieces when you use the punch, as the thin aluminium pieces need something to resist the blow, yet yield a little so that an impression is made.