Translation and interpreting are both services, and as a result are generally priced according to the amount of work involved, rather than a specific volume of output.
Conference interpreters normally charge by the day. In addition, they may have a minimum fee, will need to be provided with food and water and may require accommodation and travel expenses, depending on the location of your event. Business interpreters will charge on a similar basis, though since their assignments may be shorter they sometimes charge by the hour rather than by the day. You can expect to be charged a certain amount for "preparation" too, especially if you need your interpreter to translate materials in advance (e.g. presentations or abstracts of papers to be delivered at your conference).
Translators often assess the amount of work involved by counting the number of words or characters in the text, and applying a price per word or character (or multiples, such as per 1000 words or per standard line of 55 characters including spaces or per standard page of 1500 characters excluding spaces). Remember that the number of words in the translated document will differ from the number of words in the original document, so agree in advance whether word counts are based on a source word count or target word count. For example, typically the number of words in a German text will grow by around 15-20% in the English translation. This is largely due to the way in which German combines many words together to form larger compounds, which are counted as one word in German, whereas in English we use three or four words to express the same concept. Conversely, the number of characters is a more stable count, so a German source text of 1500 characters (often the measure of one standard page) will usually result in an English target text of roughly the same number of characters. Note that this is only true of German into English translation, however – other language combinations will produce different results. Your translator will be able to explain the differences to you. Often, translators will simply provide you with a flat fee for the entire project – after all, your accountant is only marginally interested in the number of transactions in your accounts, and your lawyer doesn't bill you according to the number of clauses in your contract, although both measurements will factor into the amount of work each professional puts into each project.