Id say about 300 to 400 quid is about right, the parts themselves will make up nearly half the cost. Plus the cost of coolant and a stat, sealant and maybe some perished water hoses you may encounter whilst in there.
If you are having aftemarket parts be VERY CAREFUL with the part number of the water pump, as the Z31 and Z32 pumps both fit, However the Z31 has less impellors on the back and will not move as much water.
Part numbers which are ok are; QH QCP3343, ADL Blueprint ADN19135, BGA CP18482, Nipparts N123031
Part numbers NOT TO USE are QH QCP2831, ADL Blueprint ADN19112, BGA CP18116 and Nipparts J1511015
If you are getting a little garage or a non zed specialist then please ensure they are using the right bits.
If your average parts monkey goes into a parts system at your average parts shop and bangs in a water pump for a 1991 300zx they will come up with a Z31 pump, the majority of garages will normally just fit it because it fits. It will go on, and it will work......just not as well as it should!!
Hope this helps.