Yep, that'll be me.

If cycling is costing you more than a few pence a mile when calculated over an extended period, you're doing it wrong. Bikes can be bought cheaply, even high quality ones. Refusing to pay more than £100 doesn't mean you have to ride a horrid BSO, it just means you have to find the bargains. DIY maintenance is cheap if you run durably engineered bikes, and your tyre wear rate ought to be no more than wearing out the leather on your shoes.
I've just fitted a salvaged tyre and salvaged inner tube to a salvaged bike. I reckon it's probably got at least a thousand miles left in it before I will need to fit yet another salvaged tyre to it. Despite my ultra frugal cycling regime, my main reason for riding is not to save money on travel, I just don't see any reason to waste money on cycling any more that I will waste money buying a new highly depreciating car every couple of years.