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Veterinarians vary on how much surgeries cost. A lot depends on how big the tumor is that is being removed, if a drain needs to be inserted (if the tumor is large and there is a large "pocket" when it is removed, a drain is often necessary for a few days), etc, etc. You should contact your regular vet for an estimate.
At age 8, in my practice, I would require bloodwork to check liver, kidneys, hematocrit, etc prior to anesthesia. My surgical patients all get IV catheters and fluids during the procedure. Pain medication afterwards and to send home is always done, too. I would guesstimate the range for surgical removal of a lipoma to be somewhere in the $400-$800+ range. But so much depends on location of the tumor.
So call your vet and ask them for an estimate. Your vet may need to see Murphy again to get a better idea of the size and scope of the surgery.