Im no financial expert either but here is how I see it.
75 extra seats per day equates to 27,365 extra possible passengers per year to Manchester, who will go on and spend money in Manchesters taxi's, shops, restaurants, and other public attractions around the North West of England.
It is also a possible 27,365 extra passengers leaving Manchester, spending money in the terminal and travel agents for people wishing to travel to all corners of the globe, via Dubai.
This is how I see it. Any extra seats coming into MAN from whatever destination...has to be a good thing for the local economy