how much do they charge for one piece of luggage and what is the weight limit?

Depends on the airline, every airline has different policies and charges for checked luggage.
This link includes a chart listing some of the US airlines policies and charges on checked bags.

http://www.thetravelinsider.info/travelaccessories/airlinecheckedluggageallowances.htm

or refer to the airline's website. You will find your info under Information and baggage guide.

Golly, it would help a ton to know who you mean when you say "they". We aren't mind readers - or at least, I am not.

Is it a bus line, an airline (which one), a taxi, a porter?