The question was about minimum price for a room and most answers have been about minimum charge.
In the great scale of things and at the end of the day the true 'result' is the percentage of profit.
You can maybe get away with charging highly for little work or charging little for much work, it all depends on how much getting the work costs.
If you are needing to spend say £25,000 per year on marketing to turnover £50,000 (I know someone who did) then you are not charging enough, your marketing is not working or you are not converting enough quotes.
If on the other hand your marketing spend is only £250.. to reach the same £50,000 turnover (as I did the same year) then you don't have to concern yourself too much about marketing or minimum pricing.
So what is the big difference between the two business - the first was fairly new start up, so needed to market heavily to find clients, whilst the second was long time well established with a good repeat business and most new clients generated from referrals.
The point being that people on this forum are all at different levels in their business's so thoughts will differ.
It all depends on the type of person you are and what you want from your business.
If your customer is happy with what you charge and the job you have performed, then you have succeeded so long as you have made sufficient profit for yourself.
To answer the question my minimum is £55.. which would cover a small lounge. For attempting a small stain for a repeat client, I would do it for free when passing, or even a new client if it was nearby and I had the time.
My marketing costs by the way are still minimal.