Don't know about prices now, but early 90's when I lived in Devon, we did 4 or 5 caravan/camp sites & we charged about £30 for carpets & £20 for seating. Also £30 for powerwashing caravan exteriors.
To be honest, generally there is not that much carpet and most of the upholstery is flat - I would estimate a 2 bed at around £85 for both carpets and upholstery as timewise you are looking at less than 2 hours. Plus, if you are on a site and have more to do, there is no extra tavelling time.
I used a portable at the time - just left it outside caravan, worked with very few problems. For anyone doing it today, I would recommend a TM, standard floor wand, hand tool and stair tool for nooks & crannies. It really is easy money if you can get a number to do. Also these jobs on sites are done before the season starts and can give a good early boost to the year.
Jim