so I've pondored the actual way to formulate an over enchanted items worth for a while and i found some information and added in some of my own experience with the subject and come up with this...
I had learned somewhere that after +3 your chances of success at enchanting go down 1/3rd successively each time you try to oe... so +4 would be 66% and so on. I think thats pretty close but i imagine that +4 is a little higher then 66%.. so i rounded it up to 70% and made this little chart for success rates for OEing..
+4 - 70%
+5 - 46.66%
+6 - 31.11%
+7 - 20.74%
+8 - 13.83%
+9 - 9.22%
+10 - 6.14%
+11 - 4.11%
+12 - 2.7%
+13 - 1.8%
+14 - 1.2%
+15 - .81%
+16 - .54%
Now to figure in the items worth using this chart... well you would simply multiply your weapons worth by the difference of success for each oe... so for instance say i had a +4 emi bow. Its base value is around 12 mill on my server you oe it to +4 so you do 12 mill * .3(the difference from 100% success of the +4) and you get 3.6 mill add that to the weapons value + the cost of the scrolls and your bow is worth 18.6 mill assuming you paid 12 for the bow 750k per enchant scroll and added in the 3.6 mill for the difference.
Now lets say you OE'd a lance to +6. On my server a plain lance is around 28 mill so you would take that then add the .3 difference(8.4 mill) then again at the +5 which is .533(14.9 mill) and again at +6 which is .689(19.3 mill) and add the scrolls value 750k * 6=4.5 mill so a +6 lance is worth 75.1 mill with this formula.
Now for something a bit trickier... I have a +11 BoP on sieghardt.. so figuring that i would take the difference in success all the way to +11 and add the EWB scrolls value (8.25 mill) and assuming a plain bop is worth 35 mill I would come up with a value of around 252.335 mill for it and that is not counting the QR SA(which would be like an aditional 3 mill about) So I hope this can help people determine their OE'd weapons and armors values better now.