The scope of work is unclear from your example. The sheet you linked to has two different sized sprites, plus a couple of portraits. There's also a note that says this is one of the smallest sprite sheets in the game. Taking an estimate to produce something similar to the linked sheet and assume the labor is the same for a larger sheet or more complex character would wreck your budget.
Hmm. This kind of question comes up often enough that we should probably create an guide for art direction when you don't have an art director. Something that lists some of the choices and rough estimates on how they impact the labor cost. For example, using unique death animations for each critter vs a simple fade out effect. Hiring a single artist (better consistency, longer time) vs a small crew (coordination efforts cost more, assets produced faster). Maybe a sample of low quality, medium quality, high quality versions of the same sprite and relative labor spent.
And by 'we' I mean one of the more experienced folks around here. I understand project management, but I don't have any experience as an art director (and my art skills are poor).
Tourist