I contracted to a large US music merchandise licensing company in 2003. At that time the average industry contract gave 30% of merchandise to the artist (even those with shitty contracts) - Britney Spears shows regularly netted $300,000+ in merchandise for two shows a night.
Using that number for the US today - buying one $20 t-shirt at a show nets the band/musician approx. $6 minus 10% / 60 cents for manager, agent, roadies etc if they have them.
By 2010 figures that is the equivalent revenue for 18,500 plays on Spotify.
One t-shirt - 18,500 plays.