That is a hard decision mate. If its low grade, there is a minor possibility of it getting better. Its not common though, once the signs start showing. Often the trip to the vet kicks in some adrenaline making them try things they wouldn't at home.

Personally, I would do the serum if it were my animal. $550 isn't bad, our starting cost is generaly $1000 at the specialist centre!! And when I was up there it was averaging around $500 last year.... so sounds about right.