In his letter, Darcy writes:

My father supported [Wickham] at school, and afterwards at Cambridge; -- most important assistance, as his own father, always poor from the extravagance of his wife, would have been unable to give him a gentleman's education.

Shades of Mr. and Mrs. Bennet (except that Mr. Bennet was better able to stand up to his wife than Mr. Wickham Sr. was to his!) So it wasn't just a small income that prevented Wickham's father from paying for his schooling. I wonder what happened to Mrs. Wickham. She must have lived long enough to continue to influence the expenditures for her son's education for most of his school years, I think, but I'm assuming she died before her husband, since there is no further mention of her.