Which compensation models are most common?
It depends on the language. For my language pair, French and English, payment is most commonly per source word for translation and per hour for proofreading/editing. Other language pairs may use per character, per line or per page.
What are the cost factors that you use to calculate your price?
I don't consider any cost factors because I don't use a cost-plus pricing model and I don't think any translators do. A cost-plus pricing model applies to merchants who are buying and selling others' work for which they markup the cost, not to professionals who are charging a rate based on their qualifications, training and experience. I set a rate which I believe fairly represents those factors in my case, and I seek out clients who need my expertise and are willing to pay the price I set.
And how do you calculate prices for reviews/proofreading?
Since the input (translated) text for a translator doing proofreading/editing can vary wildly in quality, most translators insist on seeing the translated text first before quoting. They will then estimate the time required and multiply the number of hours by their hourly rate.
For example, a machine translated text is often of such poor quality that it needs to be completely retranslated, while a target text from a highly skilled translator should not need any revision at all.
I'm not sure what you mean by "reviews" - are you referring to source/target text comparison? Proofreading consists of checking the target text for grammar and spelling errors, without reference to the source text. Editing consists of polishing the target text for best flow and readability, without changing the meaning. All of these functions are generally charged by the hour, based on the quality of the translation and complexity of the text.
Also is there any generally accepted guideline for what counts as a word and what does not?
As Susan Welsh said, it depends on the software being used and ultimately by agreement with the client. Calculation by Microsoft Word or SDL Trados Studio are most common, but there are other models in use as well.