When I started doing tri's I bought a Specialized Dolce, which has a women-specific frame. They come in pretty small sizes. It was about $700, which left me room for tune-ups and to upgrade the components later when I know how to ride. (remember the total price will creep up once you've purchased the extras - bottle cage, maybe a flat kit or wedge pack, possibly clip pedals and shoes . . . it adds up).

One thing I learned from buying my first road bike - I had never ridden with drop bars before and wasn't comfortable with them for a while, so I road more upright as if on a MTB or hybrid - long story short, now that I have "road bike posture" I find that the frame I bought feels too small. If I had it to do over again I'd buy one with more distance from seat to handlebars, so I could ride more horizontally. So this is just something to think about - spending some time on a road bike (more than just a circle around the parking lot) before buying one.