Substituting ingredients is a skill, which takes some knowledge and practice. However the more you do it, the better you get. 

Five ingredient substitutions I regularly make:

1. Basil, coriander and mint. Rather than buying several bunches of herbs and only using half of each, I regularly substitute these three soft herbs for each other. Many recipes which require basil also work with either mint or coriander and so on.
    
2. Barley, rice, quinoa and other grains. Rather than stocking every grain possible, I tend to only have a couple in my pantry at a time. At the moment it's barley and basmati rice, but once I've used these up I might buy quinoa and brown rice, gradually rotating through the grains over time.
    
3. Preserved lemon and fresh lemon. If fresh lemons are out of season, I always have preserved lemon. The preserved version does have a different flavour, however it often works just as well in dishes where some citrus is required.
    
4. Greens. If a recipe calls for English spinach and I only have silverbeet, rocket, beetroot greens or endive in the fridge, I'll use that. This will change the flavour slightly, but I rarely find this to be a bad thing. And I prefer to use up the greens I have, when they're at their peak of freshness.
    
5. Pepitas, almonds and pine nuts. While I like pine nuts, I find them expensive and either pepitas or almonds usually make a perfectly good substitute.

How do you save money in the kitchen?