I’ve found that you can save quite a bit on some helmets by buying an unpopular colour. The Specialized Airnet I bought recently in Matt black was about thirty quid cheaper than the same in white or blue/white.
I bought last year's color of my Bontrager Stavos on the clearance rack at my LBS for half off! Not the color or style I would have chosen but it fit the best and is well constructed.
I very much agree with everyone, try them on first! It is so hard, even within the same brand the same sizing can fit differently (I could not wear a Bontrager S/M in the one model I tried because it was too small, but the one I ended up with was a Small that felt bigger?!) I must have tried on a dozen helmets and was going to give up before I walked past the discontinued table.