It depends if you want beds in the GH or are going to have a hard surface throughout. Just having beds is going to be cheaper but you'll still need a path down the middle.
I opted for a bed down one side and hard surface on the other for a potting bench.
I used scaffolding planks to form a frame and laid flagstones on levelled builders sand. Then inside made a bed down one side with a gravel path in the middle, framed with wood and laid on weed membrane.
I enlisted the help of my brother for advice, he's a builder, so was told how to lay the flags and to ensure the surface was level. I live on a hill so needed to build the ground up on one and lower it on the other.