Liz, have you considered another prime? Are you looking at full length portraits or headshots? I don't shoot portraits a lot but the shots I get with some of my adapted lenses seem quite good to me. And a lot of these lenses cost well under your price range. The trade off is you have to manually focus.

I've been playing around with my youngest using a Contax Zeiss 100/2.0 lens and it seems to do a great job. I've got a Leica 90/2.8 and 135/2.8 I jsut picked up for $300 for both headed my way also. Can't wait to shake these out. But also a little google seaching will yield you lots of dirt cheap lens choices. In fact, for portraits some prefer a softer lens as more flattering. The EOS mount is very easy to adapt to a whole multitude of off brand quality glass. Here's a sample taken with my youngest sitting on the floor by an open window that provided the only light used, remember I am not a portrait shooter and this was just a quick grab shot. Good lighting and knowledgable technicue should yield even better results.



And here's a totally unedited 100% crop from a RAW File taken with the mentioned lens:


I should mention that this lens is in the $600 -$800 range for a used one but similiar lenses that are a bit slower (f/2.8) and just as sharp are plentiful. In fact my $200 Contax Zeiss 85/2.8 can easily give you comparable results.