I heard a story on NPR about this and they explained why it was so much cheaper. First, the Indian probe was placed in an oval orbit as opposed to a circular one that the U.S. typical uses. The oval orbit is achieved using considerably less fuel than a circular one; I think they said 50% less fuel. That means a big reduction in weight for the probe, which means a big reduction in the original weight for the rocket. However, this means the probe is only "near" Mars a fraction of the time during the one end of the oval and isn't as productive as a probe in a circular orbit.
Second, the labor is a lot cheaper in India. The aerospace engineers there are paid about $1,000 a month.
Third, the guy being interviewed by NPR said he suspected the reported cost was probably a little "fuzzy". He suspected some interesting accounting was used to achieve the $74 million number. He didn't think it was that far off, but he thought the cost was higher than reported.
All that said this was still a huge achievement for India. The expert also speculated that they would try to put someone in orbit at some point.