Nail bar niche, what can I charge?

I wish you good luck with it, but my guess is that you'll discover that their takings aren't what "reasonable" depends on, from their perspective: it'll be simply your provable traffic/circulation. That's what business owners generally base their "trying out advertising" decisions on - the ones who are willing to try something new at all, that is.

First off, enlighten me a bit. What exactly is a "nail bar"? When I read the headline, I first thought of a tool used for extracting nails from wood...

As Alexa said, the important thing isn't going to be how much a bar takes in that determines how much they'll pay for advertising. It's how little they can get you to accept, as well as how little they can spend to get comparable value (in their minds) from another outlet.

You could pencil in £50/month on your rate card, but expect a lot of negotiation. One thing successful bar owners learn quickly is that you never pay the published rate for advertising. It's one of those ephemeral commodities, like hotel rooms or airplane seats. Once the calendar ticks over, the opportunity to earn revenue from that day is gone. An empty hotel room is an opportunity wasted, if you could have gotten something for it.

Same with a ranking web page you want to rent. Every day it's not rented is an opportunity lost. Set your desired rate, and keep a rate below which you will not go in the back of your mind. Then roll up your sleeves and negotiate.

So would a "nail bar" be the same thing we call a "nail salon" here in the States - a place women go to have manicures and such?