Cheap but effective alternatives to Roundup

If you want to kill everything then glyphosate formulation from one of the sheds chosen for the highest concentration per unit cost. Do it very soon as the grass will stop growing shortly and it won't work so well.

Grass is exquisitely sensitive to glyphosate but some weeds like buttercup and ivy can shrug it off.

Do the sums. I reckon to kill a lawn of that size with salt in the UK would cost you about an order of magnitude more than the glyphosate (approx material cost Doff 1L glyphosate £10/400m^2) or 63p for 25m^2.

And afterwards you would have to wait for the salt to wash out too.