Few start the forum with free forum software then convert the forum to paid as they grow. Few start the project with paid forum software. So in which category you fit?
I start with free and if i see a good growth and need in software change i switch to paid. Else i stick with Free forum software.
I've always stuck with free Forum-Software. Primarily SimpleMachinesForum(SMF). I understand that there are some Companies who make Forum-Software and of course, they have to make a Profit to pay for their Employees and Facilities. But for me, I don't really like that there are some Forum-Software that you have to pay for.
I prefer to stick to Open-Source and Free Forum-Software as I find them a lot better. Not because they're Free but simply because there is a lot more Modifications and Themes to things like SMF in my Opinion.
I used both also 🙂 I start off free to get things rolling then paid version down the track. SMF was my first then WordPress. IPB is what I love using but comes at an cost. But you can get allot of free options
Sticking with free software is fine - assuming you're good at Do-It-Yourself (DIY). Well, having the talent will sure save you a lot of cash. However, though, I think I might prefer XenForo - even if I had such knowledge, simply cause I like the XF themes.
Oh, what software did I start with? Well, I started with phpBB - but I was discouraged cause I couldn't understand spambots. I mean, I was a young web developer then. Anyway, I started on phpBB - moved on to SMF - cause I thought it stopped spambots better - but I wasn't really understanding the problem.
O.K. I only recently moved to XF (the only paid solution I've ever used) around 2015 - and have generally only used that.