Many questions before pricing these: Do they have arm rests? Do they have a fabric back? ASre there "coffee stains" between the workers legs? (yes, they put the cup between their legs and it spills out on the seat.....or else something else is leaking??) Are they wool? Are they vacuum formed and glued to the foam? Those can loosen up the fabric, especially if they are well worn chairs.
I've got a job lined up like that, and some of the chairs are lightly soiled, some are filthy. I think I priced them at $8-10 a chair, which includes wiping down the plastic and metal parts. I will use 2 upholstery tools and 3 workers to do the job.....2 to clean, one to move chairs and prespray and brush. All the chairs will be located in one large conference room by the office staff.
Remember, in most cases, each employee wants THEIR chair back, so be prepared to mark them. In my case, they all want their OWN chairs back. Not someone elses.
$30 a chair would be great, dealtime. For value comparison, office chairs can sell for well over $100-150 a piece, so they should be maintained regularly.