Masquelier may or may not have been a top Belgian maker, I have no expertise in this area. But, a fundamental principle in gun valuation is "judge each piece on it's own merits, not on the maker's name/reputation". What we see here is a standard field grade double in "good" condition. It was never a high grade piece and it now displays damage in several areas, as has already been pointed out.
Having said that, it is likely a more than adequate field gun. The Greener-style cross bolt and side clips suggest a solid lock up. The double triggers and straight grip also lend themselves to a more than serviceable upland gun. Considering all of the above, if I were in the market for an upland gun, I'd pay between $450-500 for one such as this, assuming the length of pull from the front trigger is at least 14 3/4" (it appears in the images to be fairly short).