Up to you if you want a 'cheap' pair, recommend take your helmet along when you go to get them and get a either a robust frame or flexible frame to wriggle in over your ears! Always good having a couple of pairs if you can afford it. I've got small framed lenses for helmet wear and slightly larger pair for none bike riding.

Best advice? Remember to take them off before removing full face helmet - can be painful physically and fiscally if you don't!

BB