what is the cost of ipad mini 4 battery out of warranty battery replacement

my iPad ( MK9P2LL/A - iPad Mini 4, WiFi, 128GB, Black) continually shuts down and displays the empty battery icon even though i just unplugged power and the power icon showed 100%.

several power button cycles (hold for long 10 seconds or more) will bring it back.

when off the power tether, it will only go a few hours before the power icon get down to 10-20%

i think i have a tired battery that needs replacement. I am out of warranty. how much does it cost to send it in to apple HQ and for repair and send it back? and how long too?

thanks

Eric