Whatever you do, to go eventing in, buy the most expensive riding hat you can afford You can't tell if it is safe until you've landed on it - then you should immediately go out and buy another.
Whilst you are at it, buy a body guard at the same time. Make sure it covers your spine right down to the sacrum. Arms & legs repair - usually - spine don't repair so readily.
As for looks, well, smile when you are wearing it. Buy a pretty cover.
You ask for someone to tell you about the cheapest but maybe he/she is still lying in the hospital.
Please don't buy a riding hat because it is the cheapest.
Ask your aunt, uncle, mum, dad, brother, sister, friend female, friend male, indeed anyone else who obviously has money, to make a contribution towards your riding hat. Beg - do good deeds - earn some dosh. It won't cost so much then.