It's not a second-hand mini - it's a second-hand BMW mini, and about the only thing in common it has with the original design is the name and more-or-less the same shape. Everything else has high-end BMW specs., and that is reflected in the price. They are very popular, so their s/h value is high, hence the price tag.
Personally, I wouls pend that sort of money on a larger less-high-profile and therefore newer car, but as I said, they are very popular, and desireable, and so sell very well.