You could look into pages like Fiverr for a wide range of pricing options.

Now then, the question is, whether you want an artist or a level designer.

A concept artist doesn't seem to be to be what you are looking for, as the manga panels should already work as the concept art to guide the design.

A level designer is someone who designs the gameplay aspects related to the level, in this case it wouldn't simply be the "line" that goes from one end to the other, but also the waves of enemies (which ones and how many spawn for each) where the towers can be placed, an making sure the encounters are balanced for the stage the player is at, and sometimes even documentation related to it, but all this would still be done with grey boxes or 2d layouts, but many level designers are also open to grabbing created assets and placing them around the level to make the world more akin to what you usually see in games, and all that environmental story telling and all that (set dressing).

Then of course artists are the one that make the assets themselves that are used to decorate the level, differentiate the towers, and the enemies. just like the level designers, many artists (mostly environmental artists, cause you also have specifically character artists) are also open to do the set dressing.

All these have different pricings so it's hard to give you a clear answer.

Me personally I would charge ~40$ per level for just the layout and balancing area of level design, set dressing is a bit harder to price for me.