Dentist related question.. How much does a cavity filling cost?

The fee for an annual visit including cavity filling is €52.

There are no hard and fast prices for any medical procedure. Dentists and doctors charge what the market will bear.
However there are minimum prices set by the government which are laid down on www.riziv.be and, if you don't speak Dutch, there is a link to the French version of the site. You can always ask whether your dentist or doctor charges the so-called 'conventional' - i.e. government minimum - rate.
However much you pay, the refund from your mutuelle is also fixed by law.

I can't be much more help as my French is effectively non-existent.
However I am sure that I.N.A.M.I. is the French equivalent of the Dutch RIZIV so you should be on the right track.
It's a fact of life with pricing that dentists and doctors with a large expat patient list tend to assume that we will be refunded by our employers whatever we pay and take advantage of that - even though it may not be true.
Also, never accept 'this isn't covered by the mutuelle'. Always insist on a receipt and let the mutuelle tell you if it really isn't covered.

My experience with a "conventioné" dentist out in the sticks is that her charges aren't significantly different to the charge that a NHS dentist would have made back in the UK - and then one gets a significant proportion of that charge back from the mutuelle, so routine dentistry is a actually cheaper in Belgium.

Specialist treatment, fitting a crown for example, is more expensive here than the equivalent NHS charge in the UK (but cheaper than UK private dentists), but most if not all mutuelles will offer relatively cheap top-up insurance.

I can recommend my dentist Carine Teblick, Hulstbergstraat 27, 3078 Meerbeek, Kortenberg. She speaks English, French and Flemish. You can call her and ask for an estimate. 02/253.83.33 or 0477.505034. She specialises in treating children. That's why I go to her, although sadly I am no longer a child!

If you don't have a car, check if the De Lijn bus goes close by.

(In fact, you can ask any medical practitioner in advance for an estimate.)

The €52 is the INAMI set price. Any dentiste conventionné will charge you that price and your insurance will refund probably about 80%. Google "dentistes conventionnés bruxelles" for choices. "Conventioné" means that they charge you the inami set prices.

It's A difficult thing here to find a good dentist whose honest and upfront. I had one in Overijse knew I had no insurance lied to me about the cost of a dental implant said they were really expensive and instead charged me a huge amount well over a thousand euros for a disgusting metal thing that clipped onto to my teeth, with a false tooth on it. It was painful pulled out other Fillings and went in the bin. After one week.
She knew I had no insurance as I had told her we just moved and only after told me oh well this isn't covered by Mutelle now I ask upfront what will you charge Andy if it's too much I don't go.