Region and rust are big factors but even around here I've seen them on Craigslist from $300 to over $1000. Typically I would say $600 around here for good condition but occasionally see one that isn't a rust bucket for $300.
Are you dead set on M416?
You could make a nice camping/jeep trailer out of other military trailers like this one http://lexington.craigslist.org/grd/3958395941.html
Or non military knock-offs http://muncie.craigslist.org/cto/3989721850.html
Searchtempest.com is an easy way to check several cities on Craigslist at one time if you're willing to drive for one. Also remember to search army trailer or military trailer, not just M416. Many people don't know they have a M416 and if they don't that usually means you can get it cheaper.
97 TJ- black SE, 5-speed
RE 3.5" suspension lift with RE Adjustable Track Bar, 33x12.5R15 BFGoodrich MTs on 15x8 AR-63, Front tow hooks, offroad lights, Wilderness swing away tire carrier bumper with rack, AtoZ Fabrication Rocker Guards with tube step, cb
Black Jeep Club V2.0 # 76