If you have options, I would call other vet practices and get quotes for a spay. The vet we chose ended up quoting about $200 less than the one we typically went to. Molly was spayed at 6 months because the vet said she would typically go into heat at 7 months. When he opened her up, she was already in heat, but had not displayed any signs/symptoms. Because of this her surgery ended up costing about $50 more, not a big deal for me as long as she was safe and healthy and would recover well.
I don't understand your vet's gloom and doom about spaying her if you wanted to do it soon?