Don't waste your time with cheap ones. Remember that the helmet could potentially save your life and is something you do not want to scrimp on. Having just come back from Whistler and talking to guys who have had nasty crashes, the unwaivering advice was get a decent one, as the cheaper ones, although they may pass all the relevant standards, may not offer the same level of protection as the better ones, and is it something you really want to risk?
With that in mind, I can recommend the Giro Remedy as a decent cost versus protection purchase. To be honest, this is the cheapest one I would, and have been advised similarly, to go for.