I didn't look it up* until now. Edit- whoops- I was looking at the wrong date there. See later comments.
My catalogue has F at $35 and VF at $90 for the 1918 5 centavos. XF is $250. I know that is retail but still not bad for a buck!
It'll cost you in the neighborhood of $45-50 to slab it, after everything. I'd forget about that. Not worth it. Edit- never mind- was looking at the wrong date.
Aha! Sorry. When I came in to that page from a Google search, it had the 1915 highlighted. The error was mine, for not noticing that.
There are many ways to do it and it usually has the most dramatic effect on old copper; especially old copper that has been exposed to cigarette smoke. The easiest and fastest is to just use body oil as from your forehead on your thumb and work it into the coin. It will lift off most dirt and especially tars and make the coin look more natural. To my knowledge there is "no" long term damage. There can be hidden spots or stains revealed so take a close look first. Sometimes you'll find major problems like tiny holes or putty. It works on all metals but best on copper. Only use this when appropriate and never on uncirculated coins. Avoid doing it on coins with luster. Generally if a coin doesn't have a layer of something on it then this won't help. Sometimes with copper you'll uncover more serious accumulations that require a soak in olive oil. I prefer to avoid olive oil since it requires a lot of work to make the coin look natural again.
Here it is with the 1918 highlighted. So like you said. That's a nice cherrypick. And also like you said, if you sent it in, the slab fees and shipping both ways would be your overhead. Figure fifty bucks. So yeah. The 1918 is worth it.
I used to use Vaseline. Thumb it on, then wipe it off with tissue. I agree about disliking olive oil.
Interestingly I have the 2019 North American Coins and Prices guide to US, Canadian and Mexican coins. Copyrighted 2018 and the prices on this coin are exactly the same as the NGC world coin guide you linked. So the price has been the same for at least 2 years!
It's not uncommon for the Krause/Numismaster/NGC prices (which are all presumably the same source) to remain static for five or ten years or even longer.