It would be pretty close - if they had to add something, it might be in the order of 20p or whatever - not likely enough to put them off using it if they wanted to.
E.g. ,medium coffee might be £3.10; slices and muffins range between £1.75-£2.25 (just looked on the app - might be different in some locations). Or a large fancy festive coffee, which would be a treat in itself, for less than £5
Obviously if they want fancier/bigger coffees, or larger slices of cakes, then they'd have to add a bit more, but it's certainly not the case that you couldn't get anything nice if you didn't want to add money. You could choose carefully and get perfectly lovely little treat for under £5, or choose less carefully and add 10-50p, or just put the £5 towards a much larger order.
If there are Costas around, great; if not, Starbucks or Nero or any independent cafe nearby is good. I find that even if I usually go to an independent cafe, the other cards are still useful when I'm in train stations or travelling somewhere, as you can very often find one of those, so it's always useful somewhere.
But a handwritten card etc is lovely, and certainly if someone is finding things hard, then I would absolutely not want them to feel pressured into giving a gift when they have much better uses of the money - on the other hand, if they do want to give something, and would be spending that £5 on something for me regardless, then a Costa voucher is a very good use of that amount of money, for me. It will get used, not need to be put somewhere, and I'll enjoy it. But teachers truly don't need or expect gifts, much as they are appreciated if they arrive, so please don't feel that it's something you ought to do.