Ugh! Fitting a backlight to a calculator sounds like no fun at all -- those LCDs are pretty fragile.
You can get Palm PDAs with good backlights for not very much these days, and they have a calcuator -- advantage being that the 'keys' are on the touch sensitive screen so are illuminated along with everything else. And there's also quite a lot of aviation software for the things.
Otherwise the cheapest option is probably one of those Sharp $30 (ish) organisers, which are pretty crummy but do have backlights.
R