Like all business with a client base, it's customary for the receiving business to pay a premium for the client acquisition. You are handing the other designer clients on a silver platter. You are cutting the new designer's advertising, marketing, and sales costs. In fact, eliminating those costs entirely for your complete stable of clients. I have no idea how many clients that may be, but in general it takes a long time for find even 3 really good clients. You are providing a huge shortcut. These clients will at least work with the new designer once, and from there it's his/her job to retain them. Basically the new designer would need to work themselves out of these clients. That's worth a fee.
It's generally not as much as if you were to charge a client for their files, but there is often an agreed upon fee in light of the direct client referrals, associated client data (files), and future non-compete arrangement.
In order to determine what may be an appropriate fee you should calculate the annual return from each client. Because you need to know the average income each client may provide. If you know that projected income from client X next year would be roughly $XX,XXX.00 then you can negotiate that the receiving designer pays you a percentage of that amount as a "finders fee" of sorts. You are giving that new designer the potential to earn that $XX,XXX.00 without any marketing, advertising, or other client acquisition costs.
Another option is to set up a contractual royalty for any work done for these clients in the next year (or some other time frame). You get 20% of every project from these clients for the next year, or something along those lines. This avoids larger up-front costs for the other designer and allows you to profit from the hand over of files. But, not all clients will continue to work with the new designer. So this may result in less money for the files overall.
Much of this comes down to your relationship with the new designer. If it's strictly business, the fee is negotiated in all instances. If it's more of a "friendly hand off" you may not wish to charge anything. Truth of the matter is any client data (files) aren't really mandatory for most things. It merely makes future work or changes easier. So by passing on files you are ensuring that your clients are not paying higher prices to regenerate old pieces. But just because the clients aren't paying those higher prices, it doesn't mean the files lose their intrinsic value.
When my dentist went out of business, he sold all his patient records to a new dentists and sent all his clients there. I visited the new dentists once, hated him, and never went back. No other dentist needed my records to engage me as a patient. So in my case, that new dentist paid for my records and only received a single appointment (which I paid for) from that payment. My guess is in my case, he may have lost money by paying for my records. But it's his fault for being a poor dentist, or at best a dentist which didn't suit me and my needs.