Well stated but I would add that the sponge isnt even that highly priced. $1.25 a wash (8 washes), you wash your car once a week (even though most may do every other week) and that comes out to $5 a month. The sponge can last you 2 months and you only need to store it in a zip lock bag in a cupboard somewhere as oppose to finding a designated car wash supply spot in your home to store a bottle of soap, wax, sponge, and a bucket.
Its all about convenience. $10 for 2 months of wash, $60 for a years worth.. Cant get much better economically than that.
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