"The most critical resource on Mars is it's atmosphere. This is easier to exploit than any other space resource other than sunlight. No heavy duty mining equipment required. The second most important resource is water which appears to be very concentrated on a per volume basis on Mars unlike the water that may exist on the Moon. Extracting water from the surface on Mars will be far easier on the Moon due to both it's concentration and its temperature. All you need is these two resources and sunlight to produce food, fuel, and plastic. The "Case for Mars" goes into great depth about how to produce other linchpin resources for our society from materials available on Mars. I think most of the rest of this has some applicability on the Moon too but the two key resources are vastly more available and accessible on Mars compared to the Moon. "

By this measure Earth is far more valuable then Mars, why spend money getting to Mars when there is huge quantities of water and carbon on Earth?
The Moon doesn't need huge quantities of water, though to be minable it needs to be concentrated enough to be viable. Now, it would wonderful if there was millions of tons of water in a nearly pure form on the Moon, but million of tons which was say 20% concentration could be minable. If you allow the price of a lb of water on the lunar surface to be worth $100, then a million tons of produced water has gross worth of 200 billion dollars. If water is selling for 100 per lb on lunar surface, then it possible that LH&LOX is selling for $500 per lb on lunar surface- requiring electrical power to be selling for around 50-100 per kw/h. In this "environment" lunar water could be sold in lunar orbit for around $1000 per lb- or rocket fuel for about $1500 per lb, if it cost about $2000 per lb to get crew to Lunar orbit, then the cost of getting to the Moon would in the range of the current cost of getting to ISS- less than the Shuttle cost and more than the cost for Soyuz- and take about same amount of time to get to either the Moon or ISS. A facility on the Moon could last longer than pyramids and doesn't have a 50 million per year cost of reboosting & could provide less radiation exposure as compared to ISS- and other advantages. And having thousands of tons of rocket fuel available in Lunar orbit could lower the cost of getting crew to Mars.
Now if there is more and purer water on Mars then this could translate to a lower cost of water per lb. $50 per lb or less might be a more reasonable price, but there is no reason why your electrical power cost would be less. And since it requires more rocket fuel to leave Mars it will cost more money to get to Mars and leave it. The abundant of Martian water will allow farming and possibly other exports, but you need a market to sell foodstuffs to- it's not going to sell food to Earth surface markets.
The Moon could be a good place to produce electrical power- solar and nuclear and is close enough to Earth, that at some point could export power to Earth surface, this isn't really an option with Mars because of it's distance and the cost of getting stuff off it's surface (various manufacturing goods which could be energy intensive or damaging to Earth biosphere so as to favor space production- plus with vacuum you could lower costs or produce unique products.