Thanks to all features of Joomla Translate you don't need to spend money for every translation! In Google Translate quota you only pay ones ant next time if word (phrase) is already been translated - we'll get it from the cache.
Joomla Translate is an extension for automatic translation that combines handiness of:
Joomla translation module
Power of Joomla translation component
Joomla translation extension translates 98% of Joomla website (including all website content, iFrames, URLs and meta data).
Joomla translation module adds chosen languages in website's sitemap hence improve visibility and rank of translated content in search engines for other languages (SEO feature).
Joomla Translate cost-effective! He eats Google Translate quota only in case of the first translation, so the next time the word (phrase) is translated - we get it from the cache.
First cache of Joomla Translation for translated word (phrase, html piece), we created it with Translate API, and this cache created to save your money.
The second cache of Joomla Translation for translated pages, translates urls and Url map. This cache (2) created from the first cache to save your site speed. We show full page from cache without checking every HTML piece and not gathering page from cache 1 (We do it 1 time for the first visit).
When you clean cache - you clean only cache 2. Cache 1 never cleans (because it contains already translated phrases). So if you won't change the content on the website, there is no need to add new translated phrases to cache 1. Clean cache 2 only if you change the site content (or some pages).
In Commerce version of Joomla Translation module - Joomla Translate has manager for translated pages and translated URLs. You can clean cache specifically for the page which you changed.