The Weasleys were "poor" because Molly was a stay-at-home mom, Arthur worked in the most underfunded department of the Ministry (and it was implied that he didn't get along with Fudge so he was probably passed over for raises and promotions a lot) and because they had a LOT of children to care for. And they obviously weren't elite wizards so they probably didn't know about many of those potential spells you mentioned that could have made their financial situation easier.
Keep in mind, however, that poverty is relative. The Weasleys may not have been able to afford to buy anything new and shiny, but they owned their own home, were able to send all of their children to a prestigious school, and they were all well fed and cared for. They were certainly better off than the vast majority of low income muggles.
Keep in mind, however, that poverty is relative. The Weasleys may not have been able to afford to buy anything new and shiny, but they owned their own home, were able to send all of their children to a prestigious school, and they were all well fed and cared for. They were certainly better off than the vast majority of low income muggles.