How much would you pay for a Facebook app? For most apps, most people would probably pay nothing. But for some apps, such as member-to-member online tutorial services, charging could become an option. At least Tien Tzuo hopes so. The CEO of Zuora is bringing his billing subscription service to Facebook, which has more than 50,000 apps in search of a business model. Tzuo argues:

It is very easy to build an application and easy to get distribution, but nobody’s really making money and everyone is still talking about advertising. Advertising never really worked for apps. Subscriptions are the missing ingredient for people to make money.

Zuora lets app developers charge recurring monthly subscriptions for their apps or premium features. Subscriptions can be weekly, monthly or yearly, and as little as 25 cents or $1. Teach the People is already using Zuora. Tzuo is looking for five more developers to test out his beta (sign up here).

Zuora handles the self sign-ups, the billing, the product catalog, and the hand-off to payment gateways such as Paypal or credit cards. In return, it takes 2 percent of each subscription. Zuora has raised $21.5 million so far for its billing-as-a-service model.

Maybe just having the option to charge a monthly or yearly subscription will force Facebook app developers to come up with more apps that people are actually willing to pay for.