Accurate measures of inflation, one of my fav subjects. To understand why colour photos are rare even in the 70's, I have an old 1974 negative/photo holder from a pharmacy that arranged developing, with a price of £5.60 on the front (for 36 shots). Remember , this excludes the cost of the film.
The average earning equivalent of stg£5.60 today is €85, working out at €2.37 per snap (and with film cost c.€4 per shot). Hence the rarity. Go back another 20 years and you can see how colour slides were as rare as hens' teeth.