Unless there's absolutely zero identification, I'd do my best to return the wallet to its owner intact and with all cash. My reasoning is that at least here in Oakland, a wallet or money clip turned in to the OPD with no identification would simply get stolen by the police - yeah, that's how we roll. That said, in many years, the most I've ever kept in cash without any ID associated with it at all was a $10 bill I found on the sidewalk - folks just don't carry a lot of cash these days.
That said, I've found IDs, wallets, cash with credit cards, cell phones, and a host of items with some means to identify their owners. You'd be amazed what financial institutions can do to help locate credit card owners - I've called, explained that I found the card and that it should be cancelled, the credit card company/bank takes my number, contacts the owner, and acts as a liaison. Similarly, most folks can be found on Google, and I've pinged people through their LinkedIn profiles, email addresses online and Google Voice numbers. Since we're close to Cal Berkeley, the campus police department will accept lost items if they have a student ID. Sometimes if it's just a license and a pile of credit cards, I'll mail them to the person's address on the license.
I've been pickpocketed from my purse on public transit and robbed, so I try to remember the hassle and time it took to replace everything in my wallet as well as the hit from the lost cash. Plus, you know, it's the kind thing to do.