Welcome to the forum - good question! Probably like having kids, the cost per dog goes down if you have more than one because of things they can share or buying volume.
We only have one and I budgeted $100/mo for food, doctors, toys, etc. Honestly, I have had trouble meeting that but
1) he eats well.

2) I'm a sucker for toys (& he's a destroyer of toys).


3) so far, each quarter seems to bring something we hadn't counted on. Training expenses at first, then bedding that needed to be replaced, an unexpected doctor visit - that type of thing.
I think we're getting closer to it now, but it's involved me being sure to watch the expenses with him and make cutbacks where needed - just like with the budget for our living expenses.

It's really like having a child in many ways.