I'm sorry about the diagnosis, Emily. I don't have any firsthand experience with this type of cancer or using chemotherapy in dogs.
I do have a book about cancer in dogs written by Damien Dressler that I got when Dixie had her anal sac surgery. It is very informative, and he devotes and entire chapter on this type of cancer, by stage and by type, and discusses the treatment protocols for each of those. I did read the chapter, and I found an interesting section about "metronomic chemotherapy" that isn't a cure but is meant to slow down the spread, extend life expectancy, and is given orally at home with much less side effects. He does also make a brief mention that similar results have been achieved with "certain botanicals" that he doesn't elaborate on. I don't know that I'd buy it again, but maybe you could see if your library has it. Here's an internet page about the metronomic chemo by him that I found: http://www.dogcancerblog.com/blog/low-d ... iosarcoma/
A couple of things struck me while reading the chapter where he said that most dogs with this type of aggressive cancer are not diagnosed until they are in distress with symptoms such as lethargy, GI troubles, etc, so the fact that Willy feels good and isn't showing these signs, and the fact that his labs are excellent, gives hope that you caught this in its early stages and that you might have a real possibility of slowing its spread or have more time with Willy than statistics show. I surely hope that is the case.
You are correct about the price range for the ultrasound. I was spending about $500-600 each time Dixie had hers of her heart. If it will bring you peace of mind, benefit you knowing that cancer has or has not spread and affect your decisions regarding a treatment plan, then you might consider having it done. If it won't bring peace of mind or won't change your plan of treatment, then probably not. In the last weeks of Dixie's life, the cardiologist tried to refer me to their internal medicine specialist for an u/s and other expensive tests, but based on her labs and discussions with him and my regular vet, having those done wouldn't really have changed or added anything I wasn't already doing for her, so I chose to not spend the money or put her through any of the additional tests.