If you water cool the GPU, you may as well also do the CPU as you simply need to add a block to the loop 
The basic breakdown of prices are as follows:
CPU block: £45
GPU block: £50-90 (cheaper one is universal GPU block and the more expensive is full cover)
Pump: £40-70
Reservoir: £20-40
Radiator: £45-100
Fittings and tubing: £20-30
You could also go for an XSPC kit for around £250 and then add a GPU block.
Water cooling is expensive, so I suggest you try the Cardiff first and then decide if you still want to splurge on water cooling

The basic breakdown of prices are as follows:
CPU block: £45
GPU block: £50-90 (cheaper one is universal GPU block and the more expensive is full cover)
Pump: £40-70
Reservoir: £20-40
Radiator: £45-100
Fittings and tubing: £20-30
You could also go for an XSPC kit for around £250 and then add a GPU block.
Water cooling is expensive, so I suggest you try the Cardiff first and then decide if you still want to splurge on water cooling
