If you find that your eyelids do not keep as much light out as you would prefer
Or alternately, just "borrow" some of your better half's darkest eye shadow..........
But, seriously, if you are looking for a cheap set, I'd recommend the Boots the chemist travel ones at GBP 4. They're mainly polyester, so are wipe clean but aren't too warm when worn & if you want cotton then a simple wrap in a clean hankerchief (or a permanent sewn lining) can be done at minimal cost. (One of my colleagues who suffers from migraines swears by these.)
Alternatively, if you're apssing through the US or anywhere else you might find a Brookstone store they do a nice couple of options. One at USD 20 has a pocket for a gel sachet that you can heat or cool (again good for anothe rmigraine sufferer) or there's a memory foam eye shade at USD 36.
You pays your money - you takes your chance, unless you've got someone on tap who's hand with a needle & thread, in which case you could alyas try sweet-talking them into making your own custom shades.