I want to go to grad school for Applied mathematics, and I have a high GPA in my undergraduate of 3.56 in Applied Mathematics. I wanted to take the GRE. However, they are very costly around $200, which I am not sure if it is worth paying that much to take the GRE. I plan on applying for Graduate Assistantships. If I do not take the GRE, will that decrease my chances for getting into Grad school and a Graduate Assistantship even though the schools I want to apply to do not require the GRE?
If the school you are applying for does not require GRE than it is extremely unlikely that it will make any difference.
Certainly take a look at the discussion on the importance of GRE scores for graduate school admissions; as usually GRE scores most often are used to perform "cut-offs", not as a competitive measure (or at least with relatively low weight). Thus, if the school decides not to make GRE mandatory it does not make a lot of sense to submit one (especially, if you don't have one ready for other applications).
Some exceptions probably will apply if the assistantships/scholarships have a separate set of requirements (I am not aware of specific cases that would require GRE, but can imagine such to exist) or when GRE is an optional application package item and one wants to strengthen their borderline submission. Even though in the last case, I would spend more effort in writing a better statement of intent.
Even if a grad program in math does not require the subject test GRE, the fact that you will have had the sense to take that exam will be a positive indicator about your sensibilities and understanding of the grad school application game.
In my grad program (at a U.S. R1) the grad admissions committee (on which I've served as member and/or chair for 30+ years) we do not use the math GRE subject test score as any sort of decisive indicator for admission. Yes, we do wonder about an applicant who hasn't somehow realized that many programs do care about it... and why the applicant's advisors haven't told them what this little game is.
All in all, the expense of it is a reasonable investment, even if not quite in the way that people might think.
Something important is whether GRE is mandatory, recommended or not included at all as a requirement. If it is not mentioned at all, you can skip it and rely on your language and technical grades and a competitive CV and cover letter. If it is recommended, it is best to have it than not have it because it is a straightforward application filter.
Another important element is whether you intend to apply to more institutions with different requirements (e.g. GMAT). In that case it is very likely you will need to take a test at some point because it will be mandatory, but perhaps you can avoid taking both/ all. You can ask the admissions office if they also accept X despite asking explicitly for X.