I dunno...it depends on so much.
Before being pregnant, I was barely a 32 A, and now that I'm nursing, I'm a 38 D, so I definitely needed new bras. I tried using the bras I could find at La Senza, but honestly, it was so much better to buy something meant for nursing. That isn't to say I spent tons; I went to Wal-Mart and bought two nursing bras for 15$ each. As for nursing tops, I "invested" in a few from Motherhood Maternity, for about 20$ each, so the expense wasn't astronomical, but I really appreciate being able to access the boobs quickly when DD is impatient. It's also real helpful for nursing in public. Finally, I felt like everything I had in my closet was extremely tight on me (I was about 120 lbs and right now, I've still got 25 pounds to lose) and made me look frumpy. Buying a few choice pieces postpartum gave me the opportunity to treat myself and feel less flabby/shabby.
I think that it's really a question of personal preference. A lot of mamas on MDC have mentioned that they just buy a bunch of cheap tank tops, while others say they need to spend money because they can't find their size in stores.