How much does it cost to lighten gums

Have you checked with a periodontist in your area? I've just learned that this exists so it's not even a widely known procedure meaning that it's not available everywhere.

One thing to keep in mind though is that this would be considered "cosmetic" so insurance wouldn't cover it.

Your dark gums are a result of racial Pigmentation I'm guessing? That's similar to whitening dark skin and is pretty vain and useless when you think about it.

Dark Gums Treatment in Los Angeles and Beverly Hills

I have referred to the above site for this same question many times. This dentist's treatment was brought to my attention by another poster on this forum. She was very happy with the results.

P.S. Please ignore Amir's thoughtless comment above.