Congrats on your pending retirement. I think you will find some useful information on the Expat Arrivals Moving to Barcelona and Accommodation in Barcelona pages.
Assuming you would be wanting a one-bedroom apartment, you would be looking at around EUR 450 to EUR 850. This, of course, varies widely according to area and a number of other variables and is just a very rough estimate according to reports from people living in Barcelona.
Hope this helps. Enjoy Barcelona!