First of all, welcome to the forum!
Now then, I'd like to start with you saying that usually you feed your corn two mice a week. How big/old is the corn, and what size are the mice? If it's an adult corn and your feeding adult mice, that is too much - an adult should be eating one large adult mouse every 10-14 days. If you're feeding much smaller/younger mice, however, that would explain it. That having been said, this is a secondary issue at this point as overfeeding is not likely, as far as I know, to cause impaction.
As a little correction, the white powdery stuff is not powdered bone at all (they digest the bone fully) but urates. Unlike mammals, snakes pass their nitrogenous waste (urine) as a solid precipitate, rather than as a liquid. This is also secondary to the main issue however.
Are you absolutely sure that what you're seeing/feeling is an impacted mass and not infertile eggs? Infertile eggs will leave a series of defined bumps that you should be able to feel along her body, instead of a single solid mass. Snakes sometimes go off their food when they're carrying eggs, infertile or otherwise. Has she passed any stool (not just urates) at all since she last ate? From your description though, it does sound more like an impaction, unfortunately. The usual cause of that is the snake swallowing an excessive amount of something indigestible, such as the substrate or their own shed skin (snakes don't usually do this but I have one that does, and passes it intact out the other end). However, even when they do consume such things, they should be able to pass them if they are otherwise healthy and the husbandry is correct. Could you please provide details of your set-up including temps? This is more to stop it happening again or to your other snake.
Now, for the issue at hand, the first thing you should do is give her a bath. For whatever reason, the movements associated with swimming seem to help snakes pass whatever's stuck in their bowels. To do this, make sure the water is shallow and only lukewarm at most - it should feel 'pleasantly cool' to the touch, but not actually cold. I have no idea whether laxatives meant for humans would work for snakes, so I would definitely not recommend trying it. It could be very harmful to them for all I know.
If the bath doesn't pass whatever's inside, you may be in trouble. The only way of removing a very stubborn impacted mass is, I believe, surgery, and you've already said that the vets isn't an option. Some things you can try.
- Ring the vet and see if they offer payment plans. Many vets will allow you to pay in affordable installments, and honestly, if her condition is already deteriorating, veterinary attention may be the only thing that helps her. Ultimately she will die if whatever's inside her doesn't come out (assuming impaction is the problem), whether she passes it or a vet removes it.
- Ring the vet and ask them for advice over the phone. Not sure that they'll have anything to say other than 'give her a bath', but worth a shot. Who knows, perhaps a laxative would work, but you should definitely ask a professional first.
- Bit of a long shot, but around here there is a charity that funds the treatment of pets whose owners can't afford it themselves. Perhaps you could see if there's something similar in your area? Again, bit of a long shot.
- If all else fails, you might want to consider giving her up for adoption to someone who can afford to treat her. I know it's not nice to consider, but ultimately if it's the only way she's going to get the treatment she needs, it might be your responsibility to make sure she gets it, even if it's by giving her up.
Best of luck.