Prusa p3 uses direct extruder so the x-carriage is very heavy as it holds the x-axis motor. Bowden extruder seperates the motor from the hotend thus the x-carriage is lighter, minimizing the swinging during fast returns. This allows better printing. The j-head mount fits to the existing x-carriage screw holes.
I got the extruder parts from aliexpress as low as $ 7. You will also need a J-head hotend (preferably with a cooler fan) and a teflon (PTFE) tube. When you have installed the bowden and the hotend mount, before attaching the tube to the hotend, extrude the filament 5 cm and measure the actual lenght getting out of the tube ("E" in milimeters). Checkout the steps needed for 1 mm extrusion in your firmware which is about 700.
700x50=35000 steps. Then divide 35000 to "E". For example if you measure "E"=120 then 35000/120=291.667 Just change 700 to 291.667 in your firmware.
Also see my spool-holder holding the bowden extruder motor.
And also my Nozzle Duct: