It wouldn't be so much how much I spend at one time, as opposed to how much I have spent over the years in total and now occupies my toy box.
The most expensive thing I have ever bought would probably be this:
http://www.northbound.com/english/index.taf
Damn, it doesn't link. Go to shop / floggers, the one on the right.
And a few other kinky bits from Northbound (great store by the way) over the years.