I really have only just started, but had a succesfull day today! I've currently got small plastic dinosaurs which I've been leaving in the caches which are big enough. Picked up a poker chip and a small frog, which I'll pass on.
I like the idea of the polished pretty stones though, I've got a few of those I don't particularly wish to keep. I also found some really pretty buttons on Amazon which would be interesting. Personally, I don't think monetary value is the thing which matters.. it's interest and thought. I'd never pick up anything unless I had an equal (or higher) trade value item.