Outdoor steps tread advice

[kitfriendesq]

I'm looking to build some relatively low traffic stairs for the side of our house, total rise is approx 600mm, width around 1200-1300mm. 

The Peak Products Black 3 Tread Steel Stair Stringers look like a good solution but I'm trying to work out the most economic approach for the treads, but all the suitable timber treads (or timber of a suitable ~240mm width) seem pretty pricey: https://www.bunnings.com.au/240-x-45mm-1-2m-stair-tread-h3-treated-pine-domestic-use_p0090470 

1. Any thoughts on most economic stair tread material?

2. I can't find a maximum weight tolerance for the stringers but I'm assuming they won't be able to take concrete treads... 

Any suggestions appreciated!