Most of the bicycle helmets sold all meet the same safety requirements. So the differences come down to style, coolness/ ventilation, and fit. I was not impressed by Walmart's offerings, but the local bike store had decent helmets in the $20-$25 range. There's no reason you couldn't use the BMX style helmets with mountain biking. The BMX style usually aren't ventilated as good as the roadie style is all.
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"be careful this rando stuff is addictive and dan's the 'pusher'."