The way I come to look on such offers (monthly unlimited subscriptions) is not the net price itself, and not future supposed returns to it (who knows what they be, and they for sure will depend on many other things), but how many hours a week I am willing to spend on that service.

If you can and willing dedicate on average 2 hours a day (a big commitment but I think I was able to hold it for several month with them) the cost of mastering, say, Linear Algebra will be ~4 less then if you subscribe and will be spending ~30 minutes a day.