A band I used to play with recorded/mixed five songs with a fairly reputable producer (he produced Avenged Sevenfold and several other national US acts), and the cost came out to be about $2,000/song (2015). We also had all of our songs fully written out, and everyone knew their part, so the procuder really only tweaked things. However, for the amount of time we got with the producer/studio, it seemed like a bargain to me.
Recording/mixing just varies incredibly widely, and it varies by reputation of the producer, how much time, effort they put into your song, time in studio, things like that. That being said, ~$40k seems REALLY high to me (but I suppose if you are an act like the Rolling Stones, that is comparitively low).
I played with a regionally popular band around the same time and we were able to record a 5-song EP for $7500; it sounds like we got a similar level of attention from our producer (who had a few "oh yeah I know them" acts on his resume but none as popular as Avenged Sevenfold).
$40k would be extortionate for what we got, but there are producers out there who are worth it. The Beatles got a good deal no matter what they paid George Martin.
$40k a song would be for a full orchestra from a smaller state college or conservatory recorded in a million dollar venue.
I'm making this up obviously, my point is recording technology has increased vastly in quality and decreased vastly in cost, so for a single song to cost $40k just to record (assuming that mixing and mastering is done elsewhere) there has to be a LOT of moving parts and unique considerations ramping up the cost.
$40k should be enough for your average band to record a full medium to high quality album and have it mixed and roughly mastered. Not necessarily by the best in the business at any level, but you should still have a high quality, release worthy result at that price if you are not being taken advantage of.
Recording/mixing just varies incredibly widely, and it varies by reputation of the producer, how much time, effort they put into your song, time in studio, things like that. That being said, ~$40k seems REALLY high to me (but I suppose if you are an act like the Rolling Stones, that is comparitively low).