I'm assuming they'll lose money on me if I turn up for the flight (based on the assumption that it costs RYR more than €5 to fly me to BHX), but what about if I don't turn up, is that €5 profit for RYR.....?
yes
If you go, they'll have to pay the full taxes anyway so it's a loss - but they hope to be profitable anyway on the PAX number subsidies (depend on routes but maybe your destination airport is giving c10€ to Ryanair for you, so your ow taxes almost refund, still the DOC to pay anyway ;-) and last minutes booking (even if on some routes, even last minutes bookings generate very few money)
If you are no show, you can't get a refund, so it's a profit for ryanair