I think it might be hard to get realistic/sensible/accurate figures here, just because most of us ( so I'm lead to believe ) run our own biz, so our day rates would be on par with your day rate with little room for your override
Not to willy wave but 8w is well below what I'd be asking for.
You have to weigh it up does the expensive guy do more work, is it good quality work ie no snags, does he shift out a good amount a day ? If so pay him, if not get shot.
Not sure what or how to pay a subby. Who supplied all his own WFP gear, water and van, maybe £120 per day + completion bonus (£30) or 35 to 40% or the days work
Darran