Treated lumber down the middle and gravel for the rest.

sand and small pebbles. you may also be able to reuse it in your green house. it will also brake down well when you move your house.

Gravel or stone; several inches to help with water drainage.

mine have crushed gravel floors and I have found I dint need boards down the center. The gravel is good to walk on even wet and one other advantage to the gravel is you can keep it wet and raise the humidity in the greenhouse when its closed up and even somewhat when vents are open

I have a big rubber mat that is used to put on a dirt floor in a barn for horses

You can buy at a feed store. Easy to take with the greenhouse too when you move it

4 to 6 inch depth of river rock or gravel base would be the cheapest.

We put down 12 inch square pavers over that for walkways.

Only problem is that plant debris are hard to remove – you either need to rake or use a wet/dry vacuum.