You started out with saying that you are a beginning quilter. I would suggest that you not buy extra rulers until you find that you have a purpose for them. You may well discover that the rulers you have can easily serve duel purposes.
You may want to watch for rulers on sale at stores such as JoAnns or other quilt shops. I found a very large square ruler that was more than $50 + at a shop that was going out of business and got it at a HUGE discount. Don't use it often but I loan it out to trusted friends who are in need of that size for extra large blocks. Check resale type shops, garage or estate sales. Become part of a quilting group. Often folks clean out their sewing room or studio and find they have duplicates or rulers they no long use. There is your chance to get an extra ruler at no cost or at low cost.
I have a 2.5 X 6.5, 2.5 X 18 (I use a lot of 2.5 squares) a 6.5 square and a 12.5 square that are used almost daily. Those along with my BIG square are sufficient for me. I also have a couple of variations of rulers for cutting triangles but honestly I seldom use them.(I use the diagonal line on my square rulers. FYI, I
never buy rulers that are designed for one quilt project. It is a waste of my $$ if I will not use the ruler for another project. My choice. I would rather spend my money on interesting fabric or an occasional trip to a long arm quilter.
Happy quilting.