I've seen Unimog, 404, axles go for about $1500 for front and rear. You'll need a yoke, though. I've seen a few places that make them.

If I was building a rig, I'd be more into 416 axles, though. Some have F/R discs. You'll pay 2x for them, though. Still, with the 416's you have the "super fast axle" option that you will not have with the 404.

"Stubig" - '72 416.141 (U1100) Unimog
"Sassy" - '05 KLR650
"Brody" - '04 Rodeo (It's my wife's, really)
"Candy" - '02 TDI New Beetle (yes, a family of 4 fits inside)
"Twinkie" - '72 Revcon 250 (converting from Olds 455 to 6.5L Diesel)