I did it for about $100. I combined a Fieldline Tactical Alpha Ops Internal Frame Pack with the foam from an Ebay cheapie, The Fieldline back pack is rugged and has more than enough room for my needs. With this setup, I packed a P2V+ with 4 batteries, 2 chargers, 2 sets of props, a Sony HX400v camera, MacBook Air binoculars, and a lot more. I yomped this rig through 5 SE Asian countries on a grueling 3 week expedition last year..
It fits easily in overhead compartments on jet aircraft and under the seat in ATR-72s.
The Fieldline sell on eBay for less than $50.
http://www.ebay.com/itm/Fieldline-T...470?pt=LH_DefaultDomain_0&hash=item5d57d26bc6
There are numerous cheap backpacks on eBay. I wouldn't trust these for anything beyond light duty work, but the foam fits nicely within the Fieldline. These run for less than $50 also. Here's an example.
http://www.ebay.com/itm/Backpack-Ba...774?pt=LH_DefaultDomain_0&hash=item3f4c55b05e