Thanks for the question!
Are you talking side-scroller kind of sprite here? And what quality/detail level? Humanoid, or creature of some kind? (creatures are much cheaper), and what degree of stylization? If you can post a reference, that can help a lot.
There are usually three components to cost here:
1. Concept 2. 'Base' 3. 'editing'
1. The Concept:
This can be done in a sketch or in pixel art (although usually this stage is done as a sketch first)
This can take a while, or be pretty quick, depending on how particular you are about the character. If it's a main character, sometimes there are a lot of revisions, and those cost more because it takes more time.
Then if it's a sketch, it needs to be translated into pixel art. If you already have a concept, then that's easier (a picture says a thousand words), and that can be converted straight into pixel art if it's clear.
This can cost anywhere from $25 - $250 (or maybe more if you need really finished poster/cover art to show- that can be good for a Kickstarter) depending on the character, how many revisions to get the look *just right*, etc.
2. The 'Base':
This is where the animations are done first ideally, roughing out all of the frames, anatomy, etc.
You have a lot of animations (animations are the most expensive thing in 2d art: I recommend skeletal animation for cost savings if your game concept can handle it).
If you just need one character, it would be done with that one's colors, but is easily changed later.
If you need more characters, or if they have equipment, it's usually better to make a "naked" base.
This can be reused (with editing) for all of your characters that have similar animations and basic body shape. E.g. it's easy to use one base for all of the human characters in a game much of the time. Sometimes there are a couple added animations per character, but a lot of it is the same (running, jumping, falling, etc.).
This is the most expensive thing. Totally depends on the detail of the characters and animations. For what you want, it could be $200 - $1,000 (depending on if we're talking about some round kirby-like thing, or a very detailed human with predominately six frame animations)
If you need further characters, we can edit the prior character base. As little as $5 or as much as completely redoing the character, depending on how different it is.
If you just need colors switched around, that's really easy.
A gender change is a little harder, and might be $100 to a full new base, depending on how elaborate the base animations are, and the degree of dimorphism (like old school barbarian/maiden differences). Major add-ons like long hair or cape are a little harder to animate in too.
Without knowing more, I can just narrow it down to something in the range of $225 - $1,250 probably.
Simpler characters with more stylization are cheaper. NES vs. SNES colors isn't a big difference (makes some things harder if you want a detailed character).