just put ours up for the third year. it’s a bestway 16′ x 8′ one with a steel frame. for the first two years I put a layer of the halfords garage mats on top of levelled, well compacted soil for a combination of insulation and protection from stones from beneath.
there are 12 supports that can dig into the ground when it gets wet and these were squashing the mats and letting soil up through the gaps. Made cleaning the liner before packing away for the winter a real ball-ache.
This year, I put a layer of fake grass on top of the garage mats, plus 2′ all around to try and keep it clean and look much smarter. Going well so far.
the mats are frequently on special offer (2 packs for £20 or similar, sometimes a bit less). Grass varies dramatically. depending on the shape/size you might be able to get an offcut. I went for the second cheapest as not aiming for super realistic or nice to walk on etc and i thing it was about £4.50 a square meter.