Cheap Roof Needed

I am building a small shed/gym in my back garden, nothing fancy just 4 cavity block walls a door and a window.
I am ok to do the blocks myself but am just wondering if anyone has any advice on the simplest/cheapest way to roof it.
I was considering the Tile effect sheeting but am not sure if it is any cheaper than a traditional slated roof.
Any suggestions would be much appreciated!

WBP and torch on felt