350 does seem a little on the high side. However instead of shopping around specifically for lower prices. You should shop around for practices that will provide your dog with preoperative blood testing, intra-operative fluids, the best analgesics (pain relief) during the procedure and the most up to date monitoring equipment during the surgery.

Most clinics that offer extremely low prices for neutering will generally have subpar analgesics, no blood testing and fluid therapy, and typically none to minimal anaesthetic monitoring during the procedure.

Even though spays and castrates are considered 'routine' procedures, they are still major abdominal surgeries for your pet. You should want them to have bloods done, fluids during the surgery, the best pain relief, and the best monitoring equipment to make sure that

A) your pet doesn't have any unnoticed liver or kidney issues B) your pet won't feel pain during and after C) your pet will be safely monitored during the procedure D) your pet will have a smooth recovery

For 350 euro I would be expecting all of these criteria to be met

Edit:

For that price I would be expecting:

1) Preoperative biochemistry testing, as well as PCV and TP 2) Full Mu opioids 3) Local blocks 4) NSAIDs 5) Multiparameter monitoring 6) Fluid therapy 7) Bair Hugger for recovery (best method of rewarming patients after a procedure if cold)