I'm calculating fees for a game that I am in the process of making. I have no plans to announce any jobs until I have some estimated costs.

I can do basic art, like here: https://opengameart.org/content/simple-iso-city-work-in-progress (note: the game I am working on has nothing to do with this) However, there are certain more complicated things that I need drawing.

1. How simple is it for an artist to follow an existing art style?

I'm not making a city game, but for this example let's assume I am. I use my existing assets linked above to save time, how accurately would an artist be able to stick to the existing style so that their's doesn't look out of place? I'm considering to do some of the art myself for the current game to save on budget, but if it ends up looking worse (i.e. their art looks too good compared to mine so it all has to be redone) than there's no point me bothering to do any myself.

2. Speaking of which, how does mine look?

3. What other techniques could I use to reduce the amount of time (and therefore paid hours) of an artist? Or what jobs could I do to save the artist time? Would drawing the shapes and outlines of objects, and asking the artist to colour it in also save time?

4. Assuming I was doing the city builder again, and I had another 40 buildings needing to be drawn for example, would it be best to charge per building/asset, or per hour? It would take me 1-3 hours per building depending on the complexity of it. For example, a house took an hour but the coal power plant took about 3.

5. Assuming that the artist wanted to do their own art style and didn't want to carry on a style I created, would it still be faster for them if I had already drawn the assets as examples, or would they be better off without?