Yes I forgot about shipping which should be about $35 for most US states. But I did track about 50 auctions of various G4 towers Quicksilver, Mirror Drive Door, and Firewire 800 models. As well as used Quicksilver CPU modules. I was considering modifying a QS dual 800mhz CPU module into my sisters 466mhz Digital Audio G4 and wanted to compare the cost against other much faster G4's, I included MDD and FW800's for the costs of RAM and HD controller upgrades. In the end I ended up building a more modern PC H**kintosh for her

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Anyways Stock 733 QS's look to be less than $50 currently given that some listed with a starting price of $50 have zero bids. Probably $45
An 800mhz model should be about $10-15 given the value of the CPU card from them which translates well into full unit price differences.
An 867 would probably be $35 more because of Leopard minimums.
For a Dual CPU the value doubles from the single processor models of the same speed. A dual 800 with stock everything would be about $180 as opposed to a gutted one at $135. The Dual 1Ghz is the holy grail of the QS's and run $300 but at that range I would go for a Mirror Drive Door because of the faster and cheaper memory.
That is why I said $65 assuming an 867mhz within 10miles, then deduct $0.50 for each mile thereafter. You have to factor in the value of your time, 1 hour minimum. Then the cost of fuel to go there and back plus the ATM. With time and gas at 10miles you are risking at least $20-25 if you have an average MPG car and average pay job on a machine you may end up not wanting. While the risk on your money is greatly reduced by buying through an ebay seller with a good reputation.
Assuming that you buy through ebay the 867 would run about $100 shipped that you can feel fairly assured would run. While this other at $65 and 10 miles would end up costing you $85-90 with very little assurance it will run properly. So if you bought at the asking of $75 you would really be spending about $100 with little assurance of a good computer and losing that hour that could have been used for fun or an hour of overtime.
On the searches I go through the categories then select specifics like Powermac and G4 in the left hand pane. Then I type in 733 or 800 or whatever speed I am looking for and sort by time remaining. Any model at or over 1Ghz I just type Ghz or Dual since most over 1Ghz are Dual CPU. Other search variants 733mhz, 800mhz, 1ghz etc...
2001 QS's
733/867/2x800 MHz
2002 QS
800/933 MHz (rare)/2x1 GHz
MDD
2x867 MHz/2x1.0 GHz/2x1.25 GHz
FW800
1.0/2x1.25/2x1.42 GHz