I've Got tons of trophies, over 500 actually, and I've been looking at my profile, but theres no category for hidden trophies. So I was wondering what they are equal to; a bronze? A silver? Can't be gold or platinum for obvious reasons.
They can be gold or silver, or bronze, from memory, IIRC, looking back at God of war 3, and others, theres a selection of all 3.
I suppose if you want to lump them into that category, they could be assorted, and on cross platform games, they are identical to the xbox 360 equivalent's of hidden achievements, which in some cases, amount to nothing much, and TBH, some are silly pointless activities, thus making a lot of them bronze.
If you see a hidden trophy, and the title is on xbox 360 and ps3, you can check it against the 360's version, and you can see how it would grade, for gold silver or bronze, or check out somewhere like PS3Trophies.org