There is an admission fee (approximately around 500 yen) for entry. Then, you will have to order either a dish or drink and it costs around 500 yen ~
1500 yen. If you want to take picture with maids, or play games with maids, you will need additional few hundred yen per activity.
See the price for @home cafe as an example, @home cafe is one of the most famous maid cafes in Akihabara: