It was worth it to only have one or two families to deal with instead of three or four. My kids are both in the same daycare and I get no discount at all. In a home daycare much of the hassle for the provider is in dealing with parents and their schedules and wishes for their children. Therefore, caring for 2 or more children with the same parents is a lot less hassle than caring for 2 children from 2 different families. In my experience, home daycare providers do offer a break for more than one child.
Daycare centers are an entirely different story.
I don't really think that cost issues for the two can be compared. Costs at a home daycare do not tend to change as the child gets older. Well yes, most declared providers here get paid vacation days, sick days and holidays. There is usually a degressive scale for the 2nd and 3rd child but as someone said the best deal is someone in your home since they tend to charge the same for one or two children over 5 months. Legally 2.5 is the age but there aren't enough places.