Usally not a great deal more than an average spay cost. There will be antibiotics involved. Unless this is a dog you are dead set on breeding again in the future, this is the way to go. If you are trying to save the uterus, then you are looking at considerably more.
Pyometra is fatal if it can not be eliminated by medical or surgical treatment. It is unusual for
antibiotics alone to work effectively for a uterine infection. In most
cases, either surgical removal of the uterus, or treatment with
prostaglandins and antibiotics in combination is necessary in order to
resolve uterine infections. Prostaglandins stimulate muscular contractions
of the uterus and help it to clear infections by expelling infectious
material. It can be hard to be certain that a uterine infection is
present, but X-rays will show the enlarged uterus in some cases. Ultrasound
examination is a good tool to diagnose this disease, if it is available as
a test option. High white blood cell counts are usually associated with
pyometra but some dogs have normal or even low white blood cell counts with
this disorder. Increased drinking and increased urination are a sign of
bacterial toxins associated with pyometra. If these signs are present it is
very important to aggressively treat this disorder.

