Investment and probability
The simplest thing is to calculate the expected cost in the two scenarios:

Scenario 1: Develop both at the same time
$$E[cost]= X + Y$$

Scenario 2: Wait and see if task two is needed
$$E[cost]= X + 0.7Z$$

However I would not say expected value calculation represents the "economics view". The general view rather depends on your preferences, often expressed as utility functions. It could be your utility is a function of expected value, but it could also not be the case. What is your risk tolerance? Some people seem risk-seeking. Are you ok with a a small possibility of ruin (bankruptcy)? Preferences can also depend on reference points and more... 

For other simplistic approaches you could start with value at risk.