This is the simplest of ideas...you simply get a dollar store glass vase/candle holder and a bag of stones. Voila! In my local Dollar Tree store the glass stones come in marbles, flat backed, sea glass, cubes and they also have river rocks. The vases come in all sizes, though I'm partial to the short ones with the wide openings. They're probably made for candles actually...either way they work great! The stones keep the brushes in place and easy to grab. I even use one on my nightstand for nail files, pens and such.
I also found some fancy gold vase filler on sale at Target for $2 a jar. Interestingly, the glass marbles were $1 bag which filled 1 holder and the gold vase filler was $2, but filled 2 holders. So, it really came out to the same price. You could also spray paint dried beans if you had a particular color scheme to match. There's a bunch of other things you could put in there....colored sand, plastic beads, rice.....etc etc. I might even change the filler seasonally to match the decorations! How fun!