There were thousands of Belgian made guns imported in the late 1800's and early 1900's. They were made by hundreds of different small shops and there are no records or listings for most. Theate Freres is not listed in the Blue Book or the Standard Catalog of Firearms. Most of these guns are utility grade and sell for up to $500. A gun like you describe is a higher end gun and would be valued at $750 to $1200 depending on condition. The only auction listing for a Theate Freres that I could find sold for $110. There were no pictures or mention of engraving which leads me to think it was a utility grade gun.
I hope this helps you,