The Blenderfarm pricing model is simple and straightforward: You pay 9€ per hour or part of an hour.*
Use the Rendertime Calculator here to estimate the costs of your job. Be aware, though, that this is an estimate. There are variations in benchmarks for CPU speeds, and the rendering time for an animation may vary from frame to frame. So you are advised to add some extra credit to your account just in case. The excess will remain for future jobs anyway.
* This means that the minimum cost for rendering a job is 9€ no matter what you render, and you will be debited up to the next full hour.