Besides an actual body kit. There's only two options for lips on the 04-06 Sentras that I have seen. One of which is the Nismo. The other is the Ford Mach 1 lip which requires slight modification, but still looks good imo.
check it--> Where can i find this lip???? - b15sentra.net forums
Nismo is indeed expensive. I don't have an idea of what one would cost. But you should be able to call up any ford service department and ask for the price.