If the price is for a factory insulated indirect cylinder, complete with new immersion heater element, includes reconnecting all pipework and making good I'd say it sounds reasonable. The price should also include for bleeding rads, as there will now be air in the system.
Your plumber may not be able to source an exact replacement (factory insulated cylinder) to match your existing one in terms of pipe work tappings and he may suggest using an uninsulated cylinder which will then require a lagging jacket. The preferred option is to get a factory insulated cylinder and modify your pipework to match. The factory insulated cylinder is more efficient than a lagged cylinder. Firm up on the details with your plumber and ask him to look at the old cylinder and the pipe configurations so there'll be no nasty surprises along the lines of "oh that pipework is too short/ inaccessible, I'll have to replace all that etc etc" meanwhile another €150 outlay!