From Chapter 5: "Sir William Lucas had been formerly in trade in Meryton, where he had made a tolerable fortune, ... he had removed with his family to an house about a mile from Meryton, denominated from that period Lucas Lodge, where he could think with pleasure of his own importance, and, unshackled by business, occupy himself solely in being civil to all the world." [emphasis mine]

Since the Lucases have several children, I would assume there are at least 4, and a tolerable fortune might be just enough to support the family in the life style to which Sir William wanted to become accustomed. A tolerable fortune does not seem large enough to set Mrs Bennet drooling! And the boys might, in fact, be too young to be suitable as (current) marriage prospects.