From memory a typical studio musicians pay rate back in the late 60's / early 70's was around $10 to 15 / hour -- and lots of studios / producers tried to pay less than 'union rate'.
I may well be wrong with my quoted figure.

Today, things have moved on but not by that much. A typical studio musician [[not a 'name' session player) will get around $25 per hour or $50 per completed song. Backing singers getting around the same rate.

So, aside from the cost of hiring the studio, the arranger, the producer [[the cost of the engineer coming in with the studio hire cost), sessions can't have cost that much if we're just talking musician / backing singer costs.
A typical studio session would I guess have involved say 5 musicians & 2 backing singers, so that would run [[back in the day) at around $100 per hour.
I know some record guys would make use of a local 'cheap' resource -- seem to recall the likes of John Richbough / Shelby Singleton using the local music college's student band with very little cost involved [[& that could run to 20+ musicians; strings, brass, percussion, etc.).
So the cost of many sessions were kept quite low, which is probably why so many soul tracks were cut back in the 60's / 70's.
AT MOTOWN, they had their own dedicated studio band [[the Funk Bros), who were never too happy about how much they earned [[& so were always taking outside sessions on the quiet). The backing singers were usually other Motown acts [[or the Andantes).
. . . .So session costs were kept low-ish.
So how come most Motown acts owed BG & Motown loads of money for past sessions, so didn't get paid out much when they enjoyed a hit record. Was this due to a lot of 'creative accounting' ?