Any helmet will be designed to give optimal impact protection at a certain speed, there are a couple of schools of though here.
If a helmet is designed to keep you alive in a 100mph* impact then it needs pretty firm foam, the problem is in a 50mph impact the helmet isn't as good as it could be and is more likely to leave you with brain damage. A helmet designed for 50 mph impacts will save you from permanent head injury at 50mph but an impact at 100mph will leave you dead.
Which would you prefer more likely to live or more likely to live a normal life? This is why some like the American Snell standards and other prefer ACU standards, there are other differences like puncture resistance (i.e. spears to the head) but nothing really relevant here.
So buying a helmet suitable for the Manx GP may net be the best thing for using on a scooter. Unfortunately this information is difficult to find and compare but may help you understand the limits of a cycle helmet.
I imagine most cyclists bang their head at low speed, normally on the road when they fall off and as a result cycle helmets are optimised for a speed of around 6mph. It follows that in a major road accident they won't offer anywhere near the protection that many people seem to think.
The other problem with helmets is keeping them on, most people don't do up the chinstrap properly and in a proper accident they will fall straight off. It is suprising how much the head bends and I've seen video where a well attached full face helmet is forced off someone's head easily.
I also believe the "What is your head worth" statement to be rubbish when someone asks how much to spend, price is no indicator of quality or good design. Unfortunately it's very difficult to get the right information to make an informed choice.
Personally I have an Aldi helmet that I will take off if it gets uncomfortable.
Edit: Specific to cycle helmets, the more venting it has then the worse protection as the remaining foam is stiffer and applied more pressure to areas of the head.
* speeds plucked out of thin air for demonstration purposes.