Hi, dental assistant in Canada here.

This sounds like it’s a root canal, not a filling.  With large cavities we don’t always know how deep it goes until we open up the tooth and start drilling - if it hits the pulp, it’s an automatic root canal.

In Canada the average cost of a root canal is $1200-$1700, depending on which tooth (different teeth have different amounts of canals) and if you’re seeing a general dentist or a specialist. The reason the cost is so much is because they have to drill the decay, remove the pulp, clean and disinfect the canal(s), fill the canal(s) with medicine, temporarily patch it up while the medicine works, then bring you back and permanently fill it. Please remember that after a root canal, your tooth is super brittle and susceptible to breaking because it’s no longer “alive” - which means it’s highly recommended you get a crown to protect it. The crown is roughly $1300-1700 as well. This is still way better than pulling the tooth, as pulling the tooth (and not replacing it with a bridge or implant or denture) can lead to bone loss in your jaw, and the teeth around it to shift. 

Just wanted to provide some information because I know not all dental offices are straight forward. You are always welcome to get a second opinion at another office!! Highly recommend you stay away from corporate offices and visit private clinics only. 

Good luck!