How niche something is is largely irrelevant, because your userbase is self selecting and doesn't care how many of them there are.
What you should be asking is: Does the value of what you're offering justify $15/month?
Also consider doing A/B testing at different price points. Just ask for email addresses, and see if you get a different response rate at $5/10/15/20.
The parent assumed it was by month and not mentioned it being too high. So, clear indication that $15/year do not present itself as too high without any further info.
And as other commented, it might be too low. As I type this, both primary comments have very valuable advice. Listen to them and not to the fact they did not answered your proposed question.
$15/Year could be too low in fact for a niche. You may as well charge $15/Month and still get enough clients. In fact, I will say that a good niche service should be high margin because it is most likely low volume. This means your pricing should be high enough to create a sustainable business.
Going back to the Target Audience point. Imagine this. You have a service/product that for some reason can be sold to both students and large educational institutions. Would you ask a student to pay $15/year ? Perhaps you won't get much luck there. Would you be able to get $15/Year from an institution ? Most likely. Of course, sales cycle may be longer and it may take a bit more time to get the institution on board. But they may easily pay you $15/Month or even $100/Month if they get enough value out of what you are offering.
It's not even going to be worth accepting the subscriptions at that price. If it involves any amount of effort to setup/maintain/pay for hosting etc.
look for an idea that is in the $29/mo+ range.
It's more about the value than just the lowest price.
If it's too low you won't make money unless it happens to be really hands off.
I pay $20/year for feedbin.com, under a grandfathered rate. The currently advertised price is $30/year.
I'd probably be willing to pay $5/month (or more? I haven't given it much thought) for this service, though. I use it a lot, and I use it almost every day.
What are you thinking about building?
The easiest way to know if you are being too expensive(ridiculous) is to compare your own service and pricing, to the average service and pricing provided by companies offering a similar service.
I pay $19.29 annually for the Pro Finch VPN service.. https://www.finchvpn.com/order/?product=pro I don't think the pricing is high,,, i find it very fair compared to the service i get.
I can think of almost no services I pay that amount per year for. I can think of several I pay that amount per month for, though.
Of course, most of those are business rather than personal expenses.
Seem to recall my Spotify account comes in around $15 too.
Good luck.