Cost-effective way of filling this raised bed?

Looking for some advice on the most cost-effective and simple way of filling this raised bed. The internal dimensions are 2m length, 50cm width and 40cm depth. I have another raised bed which is exactly the same size on the other side of my garden. So I’m aiming to keep the cost down as much as possible whilst also having a good mixture for plants to thrive. I will be planting shrubs/perennials in them. Any tips would be appreciated. Thanks.