The cost of x-rays varies quite widely between different parts of the UK and different veterinary practices.
The actual cost will depend on whether the vet feels your dog will require a full general anaesthetic or whether a sedation would be adequate. Cost will also vary depending on how many different views are needed to establish what the problem is.
You should be able to ask your vet for a quote, as to what they would charge for certain procedures.
X-ray costs could be anywhere between £100 and £200.
I'll link you to a site where costs that have been charged are published.
I hope this is helpful. Kind regards XXXXX XXXXX
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