Melanoma removal and prevention?

One of my horses, Juliet, is a gray. I knew that they had a tendency to develop melanomas when I bought her. She has one about 1/2 an inch off from where the throat latch goes, it's about the size of a quarter but it doesn't seem to bother her and our vet has seen it and said that it wouldn't bug her. It bothers me though, I know it's cosmetic but I'm getting into showing and I feel like it takes away from her. I was wondering about the options for removal, and if anyone has had surgery done for one what the approximate cost is? Also how I can prevent more from popping up?