And beyond that, if you use a mechanical hard drive, forensics labs can actually tell what bits were written to a given sector BEFORE the current bits, because the previous bits leave some sort of magnetic bias behind. That's why military erasing standards dictate that a drive must be overwritten 7 times with random bits before the data that was on it is considered securely erased. I have no idea what it might cost to get a forensics expert to look at your drive though.
unfortunately, this is essentially an urban myth. for more information, see the epilogue to gutmann's updated paper on secure erasure from magnetic media. i don't believe there's even one reported case of data recovery after a single pseudorandom pass of overwriting. it would take something like manual scanning using an electron microscope if it were even possible at all. as gutmann wrote:
with the ever-increasing data density on disk platters and a corresponding reduction in feature size and use of exotic techniques to record data on the medium, it's unlikely that anything can be recovered from any recent drive except perhaps a single level via basic error-cancelling techniques... with modern high-density drives, even if you've got 10KB of sensitive data on a drive and can't erase it with 100% certainty, the chances of an adversary being able to find the erased traces of that 10KB in 200GB of other erased traces are close to zero.
if the data was overwritten by even one pseudorandom pass, it would take well more than the bitcoins' value to recover them. it won't be worth the time and expense.
it
is worth walking through the drive with software to see whether you indeed overwrote wallet.dat. some wiping programs don't do what you expect, either because they're poorly written or because the filesystem is more complicated than the case for which they were intended. if that doesn't work, though, you'd do better to assume there's no magical solution in hardware.