maybe i'm misunderstanding, but some web frameworks have built-in or easily-bolted-on and free user systems, like Laravel with various 'Starter Kits', or 'auth' i think is the one for django, or 'horus' and others for Pyramid, etc.

and, you can pay for boilerplate saas/subscriptions/payments functionality for most frameworks now -- some are realtively cheap ($99 Laravel Spark) or medium ($299 saas pegasus/django) and more expensive for ruby/jam/node/etc.