C.,
I have worked in the animal health industry for over 15 years. The first thing you should know is that the animal needs to be seen by a vet. Without knowing what you are dealing with, any steps you take could cause most problems, not less. I am appauled at the mama that said "most of what vets do is not rocket science..." Saying that is irresponsible. Every maggot is not the same, some can cause zoonotic diseases, and some cannot (diseases that can be transferred to humans.) The commenter who told you to pull it out with tweezers has no idea what they are talking about. With some situations you can do that, however, depending on the type of maggot, if you accidently puncture it, you may have a massive reaction in the dog that would require euthanasia or $100's more in bills.
I do not know what area of town you live in, but there are several vets that are lower cost - for example, Dr. Frank Lapuz at Northgate Vet Hospital is one of them. Many vets will work with you, such as All About Pets,Dr. Linda Zucca, in Citrus Heights. An office exam will cost approximately $35-50 depending on where you go. Once you get to the vet, talk with them, tell them you have a limited amount of money to spend up front, and they can give you some options. The biggest thing to know is that most vets get into the business to help animals, not to make money, and would rather see the animal healthy than anything else.
If you have any questions and would like to contact me directly, please do, but know there are a lot of options for you once you know what you are dealing with. For all you know, it could be anal glad abcesses, or full anal sacs. Your groomer is not a vet and cannot make that diagnosis.
Good luck and I hope whatever decision you make is one you are comfortable making.