I don't think that's a fair comparison. VPN services have significant ongoing technical costs: bandwidth, servers, labour (upgrades, support, security, incident resolution) etc which you must pay monthly but are never recovered when charging a one-off price.

If you're finding that the users you're sharing your passbox with are requesting your passwords every day, that's probably signal enough to remove them so they automatically use the service less. I'd expect the lifetime marginal cost of an additional user here to be a very small percentage of the revenue for that client.