I applied the Zener to the application since any kind of over-voltage situation will increase the voltage going to the PIC. I wanted to cap it off at 5 volts.
I'm just wondering what a voltage divider plus a zener would cost opposed to a voltage regulator to limit the input.

couldn't have the supply come in and go straight to the PIC and have the Zener regulate it to 5 volts. It has to pass through a resistor first
Yes, most zeners must have current limiting resistor. I say most cause as soon as I say all then someone will post one that doesn't.

The supply is applied between R1 and R2 and while R2 goes to ground, the other side of R1 is your output.
This sound like you're trying to make a logic gate for the PIC. There 's a good example in the PBP manual of how to do this.

Would a transistor be simpler to use in this instance?
You could use a transistor with the collector using your 5V supply and voltage divide the input to the base of the transistor. You'd still need a pull down resistor so you're not saving much. You'd end up with a higher parts count but not necessarily lower cost or reliability. Optos in essence are transistors and you've said your not having much luck with them. Though a 2n2222a and a couple of resistors aren't very expensive. If you wanted to go this route it's probably cheaper and easier to do the voltage regulator at the input.

Trying to use the same formula but solving for the other unknown variable using algebra does not work. I
I'm sorry to say math is math and it's been done here already.

Disclaimer: I've not seen your schematic and I'm often not good with word pictures. If I've botched something I apologize.