Ebay, "army surplus boots gore tex", £35 delivered.
Designed for exactly the purpose required, fully waterproof, I can't think why you'd spend more or wear anything else.
Caveat, my last pair wore through the soles after just 2000 days use for every activity from office wear to biking to hill walking in peat bogs in lashing Highland snowstorms, so if your budget is less than £7 a year you'll have to look elsewhere.
I wear them for work and they are awesome. Lightweight - feel like trainers - but comfortable and hardwearing. Mine have lasted 3 years and only just needing to get a new pair.