Yep, it costs money, but since it’s still in beta, it’s relatively cheap (cheap is always subjective, of course.)
There are people that don’t gel with BunPro, so not everyone takes to their method. I personally have noticed a huge improvement in my ability to read native stuff. I don’t think I would have gotten nearly as far in less than a year if I had been trying to force myself through grammar books, which I tend to find criminally boring.
Like with WK, having paid for a year sub means I absolutely refuse to let a day go by without using it, and I find that there is one thing that matters the most with a learning resource: you have to actually keep doing it consistently. BunPro has me showing up to practice and learn grammar every. single. day. And that is what results in progress.
Depending on what grammar book you’re considering, an annual BunPro sub will be around the same price. That would of course be more expensive if you want to buy one of the grammar books for the study path, but you don’t have to. Apart from a brief explanation of the grammar point and example sentences, they link to other grammar explanations if you want to go over tricky ones in more detail.
BunPro also has a month free trial, so you can see for yourself if you like it. ^^ Not one of those shady free trials where they make you sign up with your payment info and that you then have to cancel in time before they start charging you. Just make an account and you have access to everything for a month. You’re the best judge in knowing if you might like a product.