I asked about this on the (epic!) Galloway Walk/Run thread. There doesn't seem to be a cheap option. Most cheap is a Galloway-specific timer - google it, I think they're available on Amazon, about £35-40-ish?
As JIm says, if you have a Smartphone then an app for that is by far the cheapest option.
It's annoying that none of the cheap watches [e.g. Casio make loads of ultra-reliable kit for under £15] do this _specific_ thing, even though it would cost pennies to engineer in. I guess the demand is just too niche (once you remove the people who are happy to buy a GPS).