Getting stolen is one of the reason I didn’t buy a Brompton, that and the cost of ownership it’s a rather expensive bicycle.
That massively depends on how you like at it.
My Brompton has proved to be less expensive than my Dahon for example.
Dahon cost £900. Wheels were so badly made that they both kept needing to be trued. Each time at about a tenner a go. Think I had the front done about 6 times and the rear about 7 or 8. In the end, I realised it was never going to be ok and bought some replacements which never needed doing again. And then the frame snapped rendering the entire bike worthless in less than three years. Not only that, but the lockjaw system needed weekly adjustments to try and prevent it squeaking.
Total cost of bike around £1,150. Residual value = £50 for the wheels that I bought and now still have in the shed.
Brompton cost £1,080. I will be the first to admit it’s not been trouble free. Had a rim split on me very early (around 1,800 miles or less than a year) on which I always felt was poor - but everyone else here said was totally acceptable.
Snapped a crank - which was probably caused by a chain coming off and damaging it without me noticing. And then continued use.
Everything else has been wear and tear. Total cost around £1,150. Residual value now. I don’t know. Conservative estimate of £500.
Even assuming I hadn’t snapped my Dahon, value of that would be probably around £150. .
So the ‘cost’ of the Brompton would still be less by some £350.