I work with PixelPlow quite a lot, not because I couldn't render with my own machine, but it would be quite busy despite 8 (16) cores. A normal continuation of work is hardly possible. The cooling is another chapter. In the long run it harms to work constantly above 90°C...
So I allow myself to use PixelPlow, even if it costs (sometimes more).
Create your project according to the information on the Pixelplow site, the easiest way is a "Gathered Project".
You can only speculate about the price, because the whole rendering process is not linear. For example, if you have a lot of raytracing reflections or similar, the rendering process can be extremely lengthy. Even large GI caches have a big influence on the render time and the price. To a certain extent, you can influence the price by choosing a "slow" or "fast" processing of your order. It is not always guaranteed that your order will be rendered on multiple machines to save time. Sometimes a "wrong" setting is enough and the job is not tiled, but rendered on only one machine. Then you may have to wait a long time... (I've already waited over 36 hours), but when you let it render you have time...