Most big box sporting goods stores also sell jazz shoes that are on the less expensive side, and the dancer can then try on different sizes.
Jazz shoes usually run smaller than street shoe size, so a woman who wears a 7 street shoe might need an 8 or 9 in a jazz shoe.
Comfort and proper fit are essential, of course, since even cheap jazz shoes will become expensive if a dancer has to keep buying shoes because the others hurt her feet.
The other issue is that sizes may vary among manufacturers. A Capezio women's 8 may not be the same as a Sansha women's 8. Every dancer has to figure out the best jazz shoes for his or her individual feet, and then they can look for bargains.