Many translation agencies charge per word for translation. The simplest way to get a word count for your Gomo course, so you can get an estimate in advance, is to download the free Virtaal basic XLIFF text editor from https://virtaal.translatehouse.org. You can then unzip the the XLIFF file downloaded from Gomo and open each one in Virtaal. Choosing "Properties" will give you a word count for that file.
Each topic has its own XLIFF file plus there are some which apply to all topics such as localization-localization.xliff which you'll also want to translate. Most people ignore the localaiztion-errors.xliff file since that just contains technical warning and error messages.