When fly fishing you will need to build up over time a selection of trout flies. There are many types, sizes and colours to choose from so it can become fairly expensive to build a large collection.
If you target different species of trout you will also have to buy specifically for those species some can crossover but many are designed solely for the trout they are made for.
This along with trout damaging them with their teeth and you loosing them to bank side vegetation can all add to the cost. This is where it is beneficial to find a trout flies sale.