Ah-ha! The matting and ear infection are likely your culprits! I would still carefully look for any injuries, but I think that these are going to be your cause for the maggots.
In terms of an insecticide, any over the counter flea/tick/mite shampoo or spray should work fine. This is more to kill the eggs rather than the maggots - there's really no product designed for maggots, so one broad-functioning pesticide essentially works as well as the other. I actually just called one of my former co-workers whom I worked with at an animal shelter for a long time and this was her remedy:
Four parts of water to one part betadine (also known as povidone iodine). You can find betadine in the first aid aisle of the drug store, near the hydrogen peroxide and alcohol. You can then use a spray bottle to spritz this onto her fur, saturate it well, let it sit for 20 minutes, and then wash it off with a shampoo. If you use an anti-flea/tick shampoo, I think this will cover your bases nicely.
If you go through her fur with a fine-tooth comb once she's shaved, this will help to remove any remaining eggs as well. Shaving her is what's going to get rid of most of the maggots and eggs. And as long as we treat her ears and keep her ears clean and matt-free, she will no longer be attractive to the maggots, so that should resolve the problem.
For her ears, she will require a vet visit, as she'll need medication. I can instruct you on how to keep them clean in the meantime, to help make her less attractive to the maggots. With an ear infection, it's common to see lots of head shaking, pus, greenish, brownish, or yellowish discharge, scratching at the ear, redness, swelling and discomfort. For this, you'll need medication from your vet, but you can make her more comfortable by cleaning her ears three times a day until you can get to the vet. Timely action is important though, because you can end up with permanent hearing damage if ignored - these infections won't go away on their own.
So the first step would be to keep her ears cleaned very regularly - every day. Three times a day. If there's a lot of discharge that's forming a crust on her floppy ears, I would first wash her ear flaps with an antibacterial soap (put cotton balls in her ear canal to avoid getting soap and water into the ear). Then you can use cotton swabs and Q-tips to clean out the nooks and crannies of the ear with an ear cleaning solution. If you don't have a solution on-hand, just using a wet Q-tip will do. Just be sure to only clean what you can see - sticking the q-tip deep into the ear can do damage.
In terms of money, since she will need to see a vet for the ear infection, here are some things to consider:
Firstly, I would recommend discussing what you CAN afford with your vet. Sometimes, in the case of an animal with an infection or an injury, some care is better than no care at all, and certainly better than ignoring it or euthanasia. So if you had $50 to spend at the time, a good vet should work with you to make that money go as far as possible. In your case, this probably consists of prescribing medication for her ears (and I imagine you should be able to get an exam and meds for about $50). It's no guarantee and it's not the same as going the full gamut, which may include testing of the ear discharge to get a better idea on the type of infection, but it's usually better than nothing.
I would also call around to other clinics, as the cost from clinic to clinic for certain procedures can vary dramatically. You can ask what the fee is for an office visit - this will give you an idea of their pricing. I'm in the northeast of the U.S. and a reasonable fee is $35 here. In the south or midwest, I imagine it would be a bit less. Calling around can make a difference. For example, I had a dog with a cyst that had to be excised. One clinic quoted $800+ for the relatively simple and quick procedure. Another quoted $200 and they did a wonderful job. So that just goes to show how much it can vary.
Another option would be to contact an area humane society - like an ASPCA - to ask if they know of any low-cost veterinary clinics. Often, larger humane organizations run these clinics, or they can refer you to one in your area. Usually, they work with pet owners so that you pay what you can afford. The disadvantage is that there's usually little flexibility in terms of appointments, as these clinics are often open a few days of the week, so they're not idea for emergencies.
Another option to consider would be approaching a veterinary teaching hospital at a university in your area. Teaching clinics often charge significantly less for equal services and you'll have some of the best minds in the field working on your pet.
Here's a list of some additional resources that can sometimes help in a time of need:
American Animal Hospital Association
Angels 4 Animals
God's Creatures Ministry
The Pet Fund
United Animal Nations
This list is for local and national help resources:
I hope your dog is feeling better soon! Let me know if you run into any additional questions as you're shaving her and tending to her later in the day. And don't forget to wash her bed!