Checking the classifieds of a few old copies of American Rifleman, in the April 1938 issue one was listed for $30, another described as in excellent condition and re-blued was listed for $40, and one with the wooden holster/stock was listed for "best offer". Comparable automatic pistols like Lugers and 1911A1 automatic pistols were being listed for around $16-$20, so the Broomhandle Mausers were seemingly commanding a higher price. One was listed for $21 in a 1921 issue, and I am somewhat surprised to see the price rising in 1938. I would have thought that the Depression would have lowered the price, just as it lowered the price on many things.

That's a random selection, not an exhaustive search of classified ads in many many magazines, of course, and of course those are only asking prices: whether someone talked them down is unknown. But if you are writing a story or something, those numbers are at least plausible.