The Avalanche was the second best aluminum hard tail in the GT line up. It could polish up nicely and probably could get $150 to $200.
The Pro Flex 854 wasn't the flagship model either, I believe the 853 was. The rear shock elastomer looks like it has had better days. However it does have some nice parts and would be worth more parting out. Girvin Vector fork looks like they go for $100+ on ebay and the Syncros seatpost goes for about $40.
There are companies that make elastomers, I believe they are $40 each. The people at BB on RetrobikeUK.com I think could point you to where you could find replacements