You are correct. Then my 2 cents (whether shady or not) would be to give a value that you are comfortable with. Seeing that BTC price changes from day to day , whats it to say your BTC isn't only worth $10 USD/bitcoin (usd for example since im from USA). In theory your bitcoins could have been bought / mined when that truly was the case.
Since customs usually comes up with a value themselves, all you need to show them is a computer chip worth $X if they truly want to go that far to determine its value.
Again its not like your shipping a big device, all you are shipping are tiny ascis which are not even a finished product until assembled into a board.
If you want to be 100% to avoid any problems then for me BTC to USD (enter your own currency would be):
I will use todays mtgox value but you should use the value of BTC the day your order was purchased from Avalon:
# variables below
lastPrice=112 # last price USD on mtgox
purchase=780 # purchase price in BTC for the 10,000 chip order
orderVol=10000 # 10000 chips per purchase
chipsOrdered=500 # if you preordered 500 chips from the group buy. Enter your number here
# end variables
(purchase / orderVol) * chipsOrdered * lastPrice = Total USD value
Sample shell script if you work on a Mac/linux:
#!/bin/sh
# variables below
lastPrice=112 # last price USD on mtgox
purchase=780 # purchase price in BTC for the 10,000 chip order
orderVol=10000 # 10000 chips per purchase
chipsOrdered=500 # if you preordered 500 chips from the group buy. Enter your number here
# end variables
echo "${purchase} / ${orderVol} * ${chipsOrdered} * ${lastPrice}"
echo "scale=8; ${purchase} / ${orderVol} * ${chipsOrdered} * ${lastPrice}" | bc