If buying prescription lenses (or even not), remember there's a huge difference between a glass or plastic lens to which a tint has been applied to the surface, or a lens manufactured from material containing the tint. Ray Bans (and other quality lenses) made in this way filter infra-red and U/V light, whereas a piece of tinted plastic looks dark, but actually allows I/R and U/V through. This is much less effective and in the long term damaging to ocular health.
Also do not confuse the price with quality. There are many expensive designer sunglasses that use quite cheap lens material. If you fly a lot, push the boat out and go for something like Zeiss Umbral lenses made to prescription. I believe Ray Ban is also available in prescription.