Hey guys, my name is James and I am looking for an old pack that someone may not use anymore and can sell me it for $100. I know that’s low for a pack but if you don’t want it and CAN sell me it for $100 please contact me and send pics of the pack at [email protected]. Thanks P.S. the pack don’t need to have lights or sound so I just wanted to throw that in because that’s one of the reasons they’re so expensive.
While it’s not impossible, you will probably struggle to find a Pack being sold for $100. I would suggest either save up a little more, or for the budge you’ve mentioned, see about getting one of the Spirit Halloween Proton Pack replicas. 
The ALICE frame and webbing alone will usually cost at least $30-40. Even the time and labour involved in building a pack, even a junk pack, is worth more than $100.
You could always try building a pack out of foam-core, PVC, and found household items.