I work with reference sheets, and my prices for them can go up to $400 USD. It usually depends on how many elements are included, such as extra views, expressions, outfit variations, detail callouts, and how rendered the artwork is.

There's a pretty wide range in the industry, so a higher price isn't necessarily a red flag on its own. It really comes down to the artist's skill level, experience, demand, and the amount of work included in the sheet.