Obviously the lowest value card is the one with three deliveries that require you to build new track to a distant location to pick up the products, then build even more track to another even farther away point to deliver it.

The most valuable card is the one with a spec load on it. A spec load is a delivery that you already have on your train, that needs to be delivered to or near the city you are currently at.

But if you like to determine value simply by the numbers:
The Lowest Total Value Freight card is Number 4, the Highest Value card is number 48

The rest of the cards in order by total value and/or average value.