I'd buy the best you can. The cost of the bike is irrelevant, it's your head you are protecting.
Find a known brand, or lots of them, and try them on for fit - it should be snug but comfortable. I will only buy Arai, but that's because of the fit for my head, and I trust them - I had a very bad crash at 17, and one saved my life.
There are always sales on for decent kit, so you don't have to pay hundreds, and there are some very well respected brands, and not all are crazy prices, but there is high levels of brand loyalty.
I would have a preference in order of:
Arai
Shoei
Schuberth
Bell
AGV
HJC
Worth a look here
https://www.sportsbikeshop.co.uk/mot.../page_0/max_20