Sorry if this isn't the right place to ask, but it seems the best place to me.

I have someone who wants to hire me to do a batch of pixel art images, but they're very new at purchasing art etc. and this would be my first paid pixel art job.  So the problem is, they don't know what's a reasonable rate to offer, and I don't know what's a reasonable rate to ask.  Or what the rates are even for.  Do people usually pay per piece?  Per hour?  Per something else?  Is there some kind of standard for reasonable rates?  I don't even know where to look to find out myself!

Any information on this would be immensely helpful.

Thanks Lollige, that was extremely helpful!