airfern: Thanks for your comments guys, you have a deep technical expertise, but I still didn't get the personal answers for very simple questions from above: What would you personally do in this case?
Would you be fine to pay it, if they say "The link check up costed $300", still happy to pay? "$500" - still paying? "$1000" - just pay and move on, right?
I would pay the previously quoted fee, just like any other fee for a service that I had received.
The amount is irrelevant - you have agreed to pay for a service.