I think the rules should be fairly open, but know that there are many families and children doing this, and you need to keep the items rated PG if you know what I mean. As far as value, the sky's the limit, and that goes for the number of items as well.
The loot is always the secondary prize. The first prize is the hunt!
We placed a cache recently with lots of good stuff. Next time we're going to think of a theme. We found one that had a "Mardi Gras" theme, and it was lots of fun.
Happy Caching!