Here is a UK- based flash fiction contest where you can choose your entry fee.
They say: “We operate a choose your own entry fee system. The suggested entry fee is £7 per entry (£6 per entry for two, or £5 per entry for three or more), but if that’s prohibitive, just pay what you can afford. If you’d like to support a writer who can’t afford the full fee, why not add a pound or two?”
The contest ends 30 th November, 2019.