I think I paid about $50 for it and think it was absolutely worth it. Knowing what I know now, I'd say even the $150 full price lifetime subscription is a good value. From their website it doesn't appear that they offer lifetime subscriptions anymore, which is unfortunate. I don't know if I would pay monthly for this service.
For those of you that already purchased Brain.fm—if you prefer a separate application for music, I would checkout Fluid App. It essentially turns the Brain.fm website into a windowed application. https://fluidapp.com/