It really depends on the number of unique tiles (which, in turn, depends on the modularity of your buildings, the number of different styles of buildings, the number of different ground surfaces), the size of the tiles, the scale (larger scales mean more tiles are needed to depict the same type of object, and your examples have different scales), and whether the artist will be working from established designs or doing their own design work. And of course, different artists may charge differently for the same style and amount of work due to their working speeds or circumstances. The style and how well it matches the artist's skills will also be a factor, even in the case of those artists who charge flat fees.

Something of the scope of any one of those images would probably cost you in the low hundreds of USD. It's hard to be more specific than that, because it really depends on your exact needs and the artist you find.

Pixel art isn't a great medium for subtle textures as they tend to just look like noise, so you should focus on large-scale detail like stains and big splotches of different colours. Using subtle colour variations can create a grimy look without impacting readability. Grime in general is all about unevenness, don't just fill the walls with a solid colour. A pure solid colour just looks clean xP

I recommend filling our your hallway scene with props and then adding grime, as such things influence grime a lot and can make a hallway look gross even before you add any grime. For example, making the shadows in corners and between objects and walls deeper makes it look like the corners and gaps are filled with dirt. You could add cleaner patches on the walls in the shapes of objects that used to be there.
(FWIW I think starting with a scene and developing the tiles out of it is a good approach in general.)

Some games use a very low-opacity noise layer either built into the tiles or as an overlay, that can be good for things like concrete or a general unpleasant look. Note that this can look out of place if other art is very clean. If the clean parts of your game have a clean style, then the grimy parts should achieve grime with clean, cluster-based effects such as splotches.

"Skirt" is pretty broad. There are many types of skirts, and they move differently and affect the character's motion in different ways.

Try to find videos of people wearing the type of skirt you want (or watch people IRL), so you can learn what it really looks like. Other animations can be useful for learning how to stylise certain things, but if you only learn from other art, you'll also learn the artists' mistakes.

Have you tried using your program's ellipse tool? Circles in perspective have surprisingly "sharp" sides. Your circles' sides are very rounded and that's why they look off, they look more like rectangles with rounded corners.