I enter my collection into a spreadsheet in Google docs, then look up each game individually on pricecharting to get a solid estimate of its value, then enter it into the spreadsheet. I do this about once a year and update it as my collection grows and games rise or fall in value. A straight import from pricecharting would be nice, but I like to do it manually to account for condition and completeness. A CIB copy of Urban Champion sells for about 10-15 times what the loose cart is worth.
It's a lot of work, but I'm curious about the value of my collection and it gives me a good record for insurance purposes if (God forbid) I ever have a disaster and have to start rebuilding my collection from scratch.