Most everyone has hit on all your basics already. The only thing I would suggest is, when looking at rulers to purchase, pick up several different of all the same size and lay them all on different colored/print fabrics. See which rulers the lines are easiest for you to read. When you make that decision, buy all of your basic rulers (6/8 x 24; 12.5 sq.; etc.) all the same brand. Not all rulers are created equal! I'd also suggest getting a smaller ruler say 6" x 3" or thereabouts. Sometimes easier to trim things with the smaller ruler. And if you are using all the same brand, you know the measurements are the same.
Only other tool in the over $20 category I would suggest is a rotating mat when you have a 50% or better coupon for JoAnns or the like. Great for trimming smaller units within blocks or smaller blocks without a lot of turning of the work.