For "must haves" over $20, I agree with tessagin: buy yourself a beginner quilting set. I did that when I first started quilting & I use it to this day. It had a large ruler, cutting mat, rotary cutter & a white marking pencil in it.
But I'm going to assume you have those items and the other really basic items like a pair of sewing scissors (I mostly use the non-offset pair, but I also own an offset pair & that seems to be what most people prefer), a pair of paper scissors, some quilting needles (machine or hand based on how you quilt) & an iron since you really would be hard-pressed to try and do any quilting without those.
After those, I would say that the 2 things I really love & use all the time, since I do a lot of hand applique, are my Gingher embroidery scissors and some John James Gold'n'Glide milliner needles. If I weren't doing applique, I'd pick one of the larger Omnigrip squares & the offset pins for basting.