My cat has a scratch. Is it serious? How much is a vet diagnosis?

Thanks for using Petco Pet Education Center, formerly Petcoach! It is possible that this is a scratch. I think this most likely represents some form of a traumatic event such as a scratch like you pointed out though something like ringworm cannot be ruled out (less likely if he is very itchy as ringworm does not tend to cause itchiness). Usually, if based on further history and physical examination findings, there is concern for possible ringworm, your veterinarian would submit a culture. At the same time, your veterinarian will possibly treat for a possible bacterial infection. Either way, this would likely be inexpensive. The exact amount of money will vary greatly depending on the vet clinic. You can always check if your local humane society sees patients as they tend to be less expensive. In the meantime, just in case this is in fact due to ringworm, try to avoid direct contact with it as that is contagious to humans. It seems like you are already doing a great job by using gloves. Good luck and have a good night!