Cost? £75K for Boston Dynamics Spot, this knock off probably could retail for £10K. Far cheaper than a human infantryman
If they mount the rifle upside down or sideways so the recoil is closer to center mass, I think the recoil would have a far lower effect. They could probably change that over fairly quickly.
I also suspect that they'll end up chambered in pistol or intermediate calibers. The ammo is lighter and more compact, as are the firing mechanisms, and I don't know if these things can orient themselves accurately enough to hit something at 100 yards. Something like this that can move fast with an integrated P90-like weapon would be extremely powerful and terrifying.
You could probably build two of these, small arms included, for that 10K GBP (if you did it on the cheap).