Maybe 2.5 gp, if we consider cantrips as level 1/2 spells
In most respects, cantrips are treated as level 0 spells. There's no core rules to describe how much spellcasting services cost, but as you've identified, if you hold firm to the reverse-engineered formula the playerbase has derived from published Adventurer's League adventures, that would give a cantrip a cost of 02 x 10gp, or 0gp. That is not a useful value; despite the fact that a cantrip by definition can be cast all day without cost by whoever knows it, usually people expect to be paid for the use of their time and skills.
In 3.5e, cantrips were described as being like level 0 spells, but for most functional purposes they were considered to be level 1/2 when you needed to derive values based on spell level. If we instead take this substitution, 0.52 x 10gp gives us a cost of 2.5gp (2gp and 5sp, or 25sp). That's not a bad take for a few second's work - someone who knew a useful cantrip in good demand could cast it a few times a week and live a very comfortable and well-off lifestyle, but for most adventuring parties, it would be a really trivial cost.
There don't seem to be many cantrips that could be usefully purchased as a service, but the obvious standout is Mending, which will perfectly fix most accidental damage to mundane items. Broken gear, snapped axles, shattered priceless family heirlooms, torn dresses five minutes before the grand ball - 2 and a half gold pieces to get it good as new in just one minute seems like a pretty good deal if you can afford it. Additionally, we can compare to the cost of a skilled hireling like a carpenter or blacksmith, which would run 2gp per day of service - the cost of getting someone to cast this cantrip is comparable, which suggests that it's not horribly over- or under-priced.
If I were figuring out the cost of spellcasting services in my game, I'd probably use that derived formula as a starting point, and consider cantrips as if they were level 1/2 spells, so costing them starting from 2.5gp.