Hi there...in my office the test is about $60 plus the cost of the office visit which is about $50.
Some offices can run a parvo test in the office and have results in 10 minutes. Others have to send it to the lab and will get results the next day.
If you suspect your dog has parvo it is definitely best to go in as soon as possible. As you probably know, this disease can be life threatening. If your vet suspects parvo he or she will likely want to keep your dog in the hospital and this can be expensive (like >$1000). However, there are other things that cause a dog to vomit and have diarrhea and your vet may be able to treat your dog with medications to take home.