When I was at the VCF a few weeks ago, I had the pleasure of meeting Sellam and seeing his vast collection in his warehouse (somewhere around 1300 machines?). Despite that there are a few he still doesn't own and would like to, one of them being the Apple I. He's helped sell a few (to get fair value for the client) and mentioned (IIRC) the last four going for roughly $25,000.
I'm not sure what other systems would pull that sort of money, but it would likely have to be historically significant, have a good history for that system going on sale (like Altair #002, or one of John Blankenbaker's Kenbaks would pull a similar amount), and have to be a small production run.
Then I think without implying you're probably only asking about the personal computer market and not including a PDP-1 or Cray-1, etc.
From what I've really seen in person though the Apple-1 is the biggest cost, although there's the chance that something went for higher but if I'm not aware of the model or not looking at the moment I could have missed some unknown to me jewel.
- John