Typically this service is paid for with tips in the US. Did they overtly refuse to disclose or were they somewhat awkward about it? I'd expect them to be awkward because it is considered impolite to ask for tips.

If you have three trolleys full of luggage and need three porters, I'd probably tip each of them $20-30. (So, $60-90 total.). It would depend on how far they had to go.