is it okei to leave my dog alone in a room locked in her cage?

If you care about your dog, and I believe that you do, then it seems to me like you have two choices; find a way to treat your dog, or find her a new home with someone who will. If you can't afford to take care of her, leaving her flea infested, that is cruel and damaging to your dog. Is there no way you can get something like Frontline to treat her with every six weeks or so? Is it really too expensive for you to manage? Also, treating the dog isn't enough. Fleas can survive in carpets and blankets and things until an animal comes along to feed off of, so your dog will keep getting infested over and over again. There is a kind of spray you can buy to treat all the upholstery in your home, but again these things cost money. I'm sure this will come across as harsh though I don't mean to be, but dogs do cost a lot of money to look after, and if you're really sure you cannot afford her then perhaps you should think carefully about what's best for you, your dog and your mother.

no, that's not right or fair. this poor dog has to live out the rest of her days in a box because she has fleas? come off it. de-flea the poor beastie. something like frontline is very effective

One way or another...that dog has to live with some semblance of freedom; what's the difference between the cage in a room and the cage in a shelter? The whole house needs to be de-fleaed. Carpets, upholstery, blankets, etc...anywhere a flea can dig in and hide. It says good things about you that it breaks your heart to leave it caged up...you know what to do so...do it. Good luck.