Glass nail files might look like a bit of an investment compared to the cheap-as-chips emery board, but they're truly worthwhile.
Supposedly, glass files are kinder to the nail, as they are much smoother to use, and indeed, I notice less of a burned-nail smell when using my glass file than I do using an emery board. However, the main draw for me is the longevity you can expect to get from a glass file - so long as you rinse them every so often and make sure you don't drop them, they pretty much stay usable forever.
I've had the same glass files for about 8 years now, and they're still going strong. I have a large one in my office, for regular filing, and a little tiny one which (in theory) I carry around to deal with any unexpected splits or breaks.
The file pictured above is from Pfeilring at HQHair, and costs £11.95. eBay will find you whole sets for that price, but as usual you have to be careful that you're buying from a seller with a good reputation.