Cardamom is expensive usually, but I buy it wholesale from Southeast Asian groceries which is much more affordable.
I've substituted nutmeg and cinnamon for cardamom in the past with some success. But cardamom is a very unique spice with a unique flavor and some recipes are just not right without it.
I love cardamom in milk desserts, especially rice pudding and ice cream. It's also great in tea and coffee.