Last time I was aware this sort of thing happened, it went for £1,500+.
Yeah, it's for a good cause, but still to pricey for that tank . It's gone for insane amounts each time. So don't expect to get it "cheap". It's at $500 atm. I suspect it's gonna go up way more then that.
I paid £23 for mine back in the day. Has proven to be good value.