I've never paid for pixel art, but try comparing it with works you consider equal on the Unity Asset Store or Unreal Marketplace or whatever other sites you can find that sell game stuff like this.
Anyone who sells stuff on the Unity Asset Store or Unreal Marketplace expects their work to be bought
many times by
many different buyers. That's why the artists on there can afford to charge lower prices than one would usually charge for a one-off commission. If you're actually looking to buy art from someone who creates their art for you and no one else, then you have to expect higher prices (unless they willingly undersell themselves) than you usually see on these stores. In that sense, they don't make for a good reference point.
How much an artist will charge you for this kind of work varies significantly. How experienced they are, how fast they are able to create it, how desperate they are for the work/experience, how much they're willing to undersell themselves in order to meet an indie dev halfway.
I'm personally a 2D artist focusing more on vector art, hand-painted sprites etc. and rarely dabble in pixel art so I don't feel all that comfortable making an estimate on this. I mostly just wanted to comment that the Unity and Unreal stores can't be treated equally to paying an artist to work for you exclusively.