The air of a gentlewoman, a great deal of quiet, inactive good temper, and a trifling turn of mind were all that could account for her being the choice of a sensible, intelligent man like Mr. Allen. (Ch. 2)

Perhaps Mr. Allen wanted someone with the air of a gentlewoman, who was quiet (which Mrs. Bennet was not), and did not attempt to run things or have her own way. Mr. Allen may simply have wanted a peaceful existence and adequate helpmate.

IMO if Mrs. Allen had brought (significant) money to the marriage, JA would have mentioned it; she usually does. So I expect Mrs. Allen's dowry was unremarkably low or high.

I do wonder that they appear to have no children, as most gentlemen would wish to have an heir, but later references to Mrs. Allen's conversations with Mrs. Thorpe indicate there were no Allen heirs. I guess the Allens did not feel it important or else were unable to have children.