I think it was in Steven Seagal's first movie where he stashes a video camera with an incriminating tape on it inside the wall of his home. In a typical American home with drywall, this is relatively easy to do -- cut out a piece, place your cache inside, replace the piece and paint or wallpaper over it.
Years later after awakening from a coma, the protagonist manages to return to the home (now occupied by new owners), smash the drywall and retrieve his stash.
For advice on caching items, there is some survivalist information here:
http://www.captaindaves.com/guide/cache.htmhttp://www.ssrsi.org/Onsite/cacheguns.htm
(caching weapons and cash have a lot of similarities)
If you're handy with carpentry skills, you might consider building custom caches inside the walls, floors and even wainscoting and/or boiseries. I once shared a flight with a Montana-based survivalist who explained how his home had hidden caches of weapons - custom-built into the wooden floors and brick walls like a Scooby Doo fun house - throughout so that he could readily access his weapons as he made his way to his safe room if he were attacked.
Again, I would opt for gold coins (including those in smaller denominations) like Krugerrands, Canadian Gold Maple Leaf or US Mint American Gold Eagle bullion coins instead of cash or diamonds. Cash is liquid but if the bills are seriously out-dated, they may draw suspicion and invite unwelcome questions on other issues like tax evasion. Also, they are subject to rot, fire and not very portable. Diamonds are compact but not as liquid as one might assume.
