In a very simple example though: I'm currently considering getting a FM account, but I would only get it for the whole family, e.g. of 4. As of right now, I probably won't because 4 x $50/y is a tad too much.

So, in the one extreme, FM has to support +0 extra customers and gets $0. I'm not happy, they lose a customer, noone wins.

If I subscribe alone, FM has to support +1 customer and gets $50. Since it's their current plan, they are happy with it. FM wins.

Another unrealistic extreme, the whole family is free, FM has to support +4 customers and still gets $50. Obviously they aren't happy with it.

However, if the family gets a discount K, so that a family of four pay (K * $200), where K < 1, we get examples like e.g. they get +4 customers and +$150. If that's above their profit margins, they're happy and we're happy, everybody wins. I'm pretty sure even if K was something like 0.8 this would already attract many, the question is finding a number that suits everyone. Or it could be - get discount X% for family of size Y, so the discount is dynamic, e.g. 0.9 for 2, 0.85 for 3, etc.

I wouldn't categorize these users as "cheap" (why?) since it's not a whole lot cheaper than buying the normal plan X times. As long as there's a non-zero discount, it would be viewed differently by the users (as opposed to the current "fuck off, we don't care" approach).