Sticking with a stock AGD powerfeed body keeps cost down. As soon as you start shopping for an AGD ULE body, or one of the other aftermarket kits cost will rise a lot. A used ULE body will set you back $120-$180 from states. X-valve another $150-200 altough you can use others like RT Pro or Classics for less. Downside of Classics is the valve doesn't recharge as fast which ultimately limits ROF. The ULE (light action) trigger pull kit (£40) is also a good option. Drastically reduces trigger pull weight, and speeds up cycle time. Pneumatics parts for conversion.. maybe another £50-60. You need a suitable grip frame. Not all have room to install everthing required, and that'll be maybe another £25-40
As you can see it starts to add up pretty quickly