I've been doing a lot of senior photo shoots, and also some general portrait work over the last few years and I was thinking that a prime lens might be in order for better DOF and results....
When you say better DOF what do you actually mean? A shallower DOF or deeper DOF or a particular bokeh look? Your 70-200 is a fantastic portrait lens, it's the go-to lens for portrait shooters across the planet. I'd suggest looking at the exif data on your portrait shots and see what focal length is your most used for what you consider the best results. PS Bridge or Lightroom will sort these for you. That might help aim you towards a well informed decision for a 50, an 85 or a 135 or just to stay with what you've already got.
Looking at your current lens list, I see a gap at the wider end if you're looking for quality. You'll pick up a good pre-owned 24-105 for the $amount in your budget.
I'd like to have a shallower DOF. I know I can't get much with Crop, but using a faster lens should help, correct?
Also as I mentioned, I'll be leaving the crop world and heading towards FF around the end of the year, as I save up for the 5D3. Once I go FF, I will no longer have the 15-85, as it's for Crop bodies only. If I can collect enough, I could get the 5D3 with the 24-105 as a kit. Until than, I'd like to have a little shallower DOF in my photos.
Well, not being able to go shallow on a crop body is a myth, while indeed you can get shallower with larger formats with wider fields of view. I can get nice and shallow with a 25mm F/1.4 on a micro four thirds camera, which is about a 50mm F/2.8 equivalent with the sensor being smaller than APS-C. But, I also have a 5DII and being able to shoot wider and with shallow depth of field certainly is nice, but the big advantage is really being able to shoot stopped down and still get good subject separation. That said, my favorite is a 35mm F/1.4, but for what you do the 85mm is good, I don't like Canon's 50mm, purple fringing and bokeh could be a lot better. But personally I would go with Sigma's 85mm 1.4, sharper, far less CA, and bokeh on level with the 85L, but even a good used deal is double the price of a used 85mm 1.8