If you go on the Rated People website, you'll see that an average cost for fitting a radiator is £160, excluding the radiator and valves and this price is based on an average price quoted by 50 different plumbing firms. However in my price, I include pipework and fittings.
A radiator install can take around 4 hours to add to an existing heating system. The system has to be drained, the radiator fitted and new connections adapted to the old pipework, which is often 8mm microbore, the system then has to be refilled and every radiator in the house bled of air, plus as it's been drained, new inhibitor has to be added. If the boiler is old, problems often occur in getting it to fire up again after being drained and more often than not the pump will be a pain in the erse too.
A rad install is far from being a quick and easy job.