It may or may not be profitable, but first you have to solve what I call the "critical mass problem". I have thought about forums off and on quite a bit over the past few years. At one point, I added a forum section to my blog website I had (I still have the blog but not the forum part of it). The problem was, no-one was joining the forum organically because there wasn't really much going on in my forum section, even though I was getting visitors to my blog. So you have to have enough "critical mass" on the forum itself to make it take off. This cuts into profitability, because you either have to spend money, or time, or both, to have some fake accounts on your forum that are talking to each other to keep a sort of basal level of activity on the place until it starts taking off.
If you have good marketing skills and/or a good social media following, you might be able to shortcut or sidestep all of that and launch straight into a decent amount of signups and activity on your forum. But I think that is the exception rather than the rule.
If you do go ahead with it, I would recommend not focusing too early on how much to charge for memberships etc (if that is your monetization model), but rather to start by doing something like offering free signups for those joining by a certain date, as part of your launch. You can say something like "we may be charging a membership fee later but if you join now, you will get a free membership" (or a year's free membership no credit card required, or free membership for the first X number of people to join, or whatever it is you want to offer). Even though you think you might be losing money at the start, consider that these early adopters are saving you $ or time that you would otherwise have to spend to create an ongoing level of activity on the site.
Also there are other ways you can monetize once you have some activity (selling banner ad space yourself, also promotional posts, sticky posts, etc). As well as of course regular display ads from ad networks etc, affiliate banners etc.
No matter how much of a truly-needed unfilled niche it is, and no matter how genuinely valuable the forum will be to others, if there isn't much activity going on there when a new person comes to see it, that person won't actually bother joining. I'm not saying you should give up on the idea! I'm just saying proceed with caution. It wasn't a good fit for *me*, but then again I didn't have much in the way of marketing skills or social media following, and I didn't have that much traffic on my site anyway (altho some). I would simply say that if you pursue it, you need strengths not just in bringing people to the forum itself, but more importantly in ensuring you solve the "critical mass problem" first by having a certain level of ongoing activity on your forum. This is a bigger part of the initial investment of time/money than people might think if they haven't tried hosting a forum before.