I believe the most important factor is fit,price is unimportant as they all adhere to the same specification.
Price however gives you a wider range of sizes but if a cheapo fits better than any in the Giro range[unlikely but im just explaining]then get the cheapo one
The cheaper ranges might only come in sm/med/lrg ,whereas more expensive might come in many 1/2 sizes to get a closer fit,small, extra small ,med,large ,extra large etc etc
My last helmet was a giro exodus,which was a great fit,and my current is a £25 met that surprisingly fits better
Helmets come in head sizes in cm normally, with silly S?M?L? sizing for no reason. So measure your head
I've just got a MET Valeno SIM and it has close band sizes, this was 58-60cm (L) and I'm 59 so room for a beanie as well.
Nice helmet too, though look a little fragile for chucking in the boot with it's pointy bits ha ha
My other choice was a Giro Hex/Zen but the MET seemed to fit better.