For instance, look up a bunch of reviews, go into a bunch of commercial places and ask who does their cleaning (if it meets your standards of course). Most commercial cleaning companies also do domestic work, you can also in some instances get a single clean done, whatever areas you do/don’t want cleaned and see what the final cost is.
Cleaning company I used to work for almost always overquoted, on the grounds that we gave the worst possible out come in terms of price, and most clients were pleasantly surprised at the outcome.
There are also many who underquote to lure clients so don’t rush in finding a good cleaner.