In today's Tennessean, School Board member Mary Pierce explained why those who say charter schools are a cost to the school system are simply wrong and why charters actually save the system money (link). If you are not convinced by her argument, look at the data. The online edition of her article contained links to data that proves the point.
Below is a portion of a study conducted by the MNPS Finance Office that shows the cost or savings associated with the students attending charter pubic school broken down as to which school they would be attending if attending a district school. While in some schools it cost more to send a child to a charter school than to educate the child in a district school, in most schools there is considerable savings. Notice that for each child whose district school would be Buena Vista Enhanced Option, there is a $3,810 saving if the child is educated in a charter school. To see the complete list follow this link and this link.