How do you bill your web services?

In developing a new web service I haven't been able to find very much information on how companies bill for their web services.

Do you bill by request or only certain requests ie) GET or POST?

-would these be tracked at the application or server level?

Do you bill by bandwidth?

-again how would this be tracked on a per user basis

Do you charge a subscription to simply have access?

-this is assuming that they are only granted an api key after payment has been made.

A combination of the above or other options?

Thanks for your help.

Solution

As all things in a market economy, the price, but also the inconvenience (or convenience) and risk associated with the actual payment (irrespective of the amount) is a function of how unique and cool and valued your service or product is.

It is therefore impossible to answer the question but in very generic terms, i.e. in the form of suggestions. You actual invoicing model may base on one or several of the following

bill for a one-time setup fee
bill on a subscription basis (i.e. for a defined period, with explicitly defined maximum amounts of usage)
bill for maintenance
bill by the act, i.e. a certain amount (possibly on a decreasing unit price schedule). Such acts should be counted at the server level, (The client-side may include some audit/monitoring/log of sorts, but the server-side should be the authoritative source of info)
bill by volume (for example number of MBytes transfered etc.), this is applicable to services where there is a big variation in the volume of info produced for each "act".

In general, the price and the modality of accounting should seem fair, to both parties, particularly to the buyer, and typically, the simpler the better. The price should not necessarily be low, provided you can make the case that the service provided is effectively valuable, and that you either invested and took risk to introduce the service, or the on-going expenses associated with running the service are evident.