Karl-
Perhaps something that will help you more directly is to call all of the vendors independently. Ask them for their best "Oshkosh" price and then ask them to beat the others. Prices on avionics are almost ALWAYS negotiable, whether you're at Oshkosh or not, and it's been my experience that the listed price is rarely the price you should be paying. For example, when I was there last year, Pacific/Gulf Coast Avionics had the ICOM IC A200 comm listed on their counter for something like $799. I can't believe anyone actually paid that, though, given that I paid only about $650 from them after making them compete with everyone else who was selling it.
For you to get the information you're asking for would require someone to go to all of the vendors and then go around again and try to get them all to compete with each other. That's how you get the "deals" at Oshkosh, or the rest of the year for that matter.
I don't think you're having trouble because people don't want to help you. It's just that they can't find the real "lowest price" without a lot of legwork.
Good luck though.