How much for monthly propane service?

Rgiht now we are only able to fill out tank up half way each time and we are broke so comming up with hundreds of dollars to fill it all of a sudden is pretty hard. It's now run out twice this year. Some people have mentioned propane suppliers offer services to come by and fill your tank whenever it gets low or they are in the area, that way we don't have to remember to check it all the time. I think they charge by the month or something, anyone know how it works and how much they chrage??