My advice would be: if you do not already know of one, research (Yelp, Google, etc.) a highly regarded paint and body shop that works on motorcycle gas tanks, and/or call a couple of local motorcycle dealers and ask who they recommend.
Once you have one or two reputable shops, I'd just show them the picture and see what they think, and if they can give a rough estimate of the repair cost.
My biggest worry is there is something under the paint like rust. But even if it's just a small bit of surface rust, it may not be that big of a deal to fix (and it may not be even that big of a problem: just some spilled gas that never got cleaned up).
A good shop should be able to give you a minimum and maximum cost depending on how much repair work is needed.
I wouldn't call myself an expert (hence my recommendation to actually ask an expert) on these kinds of repairs, but--to the best of my understanding-- typically it's just sanding the bad area, prepping the exposed metal for paint, matching the paint color with fresh paint, then painting and blending it into the rest of the tank so you can't see that there was ever a problem.
