Google is ready to start charging for its App Engine cloud platform.

For the 10 months it’s been in preview it’s been free to use but limited to 500MB of persistent storage and enough CPU, bandwidth and whatnot to support about five million page views a month.

On Tuesday Google said it was ready to follow through on its intention to offer additional computing resources for a price and allow apps to scale beyond its free quotas. It said it’s been its most requested feature.

However, it’s going to lower its free thresholds in 90 days, claiming it overestimated the resources developers needed to get started. It thinks the free resources will still support five million page views a month.

Under the new regime, it says developers can set a daily budget for their apps representing the maximum amount they’re willing to pay for computing resources each day. They allocate this budget across CPU, bandwidth, storage and e-mail, and they pay only for what their app consumes beyond the free thresholds – prorated “to the nearest penny,” it says.

Mind you there are still no service level agreements to reimburse users if App Engine went down.

Google figures the resources paid for will scale to around 500 requests per second (qps) or more than 40 million queries a day, which is enough to handle traffic from being Slashdotted or Dugg. “In extreme cases,” it says, “(e.g. your application has been featured on Yahoo’s homepage), you can request additional CPU.”

Undercutting Amazon a trace, it’s proposing to charge 10 cents per CPU core hour and says the price covers the actual CPU time an application uses to process a given request, as well as the CPU used for any Datastore usage.

It’ll cost 10 cents per GB bandwidth incoming, 12 cents per GB bandwidth outgoing, which is supposed to cover traffic directly to/from users, traffic between the app and any external servers accessed using the URLFetch API, and data sent via the Email API.

It’ll also cost 15 cents per GB of data stored by the application a month and a thousandth of a cent per e-mail recipient for e-mails sent by the application.

Google warns users that they may notice an increase in the amount of data stored by their applications and listed in the Admin Console.

Seems data stored in the datastore incurs additional overhead, depending on the number of indexes, as well as the number (and size) of associated properties. It says this overhead can sometimes be significant and it’s been underreporting it.

So it’s doubling the free storage quota to 1GB.

Google wants the bills paid through its PayPal-like Checkout system (add VAT in the EU). And it says you can’t use multiple applications to avoid incurring fees.

There are reportedly 45,000 apps currently running on App Engine.

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