Raised bed soil mix on a budget

I've just built my first raised bed — 2.4m x 1.2m and about 30cm deep. Now I'm looking at how much topsoil and compost I need to fill it and the costs are adding up fast!
Is there a good budget-friendly mix I can use without sacrificing quality? I've got access to some homemade compost but not nearly enough to fill the whole thing. Would love to know what others have used.