Multiplayer won't take any monetary resources from the SP experience. They get a certain budget for the single player part of the game and another for the multiplayer one. On the other hand, it might take some of the human power, time and the attention of certain people that are responsible for a larger part of the game ( like a developer or map designer or whatever ) .
If they really want to add multiplayer to the game they shouldn't repeat what ME3 did. Make what you want with the MP part of the game, but keep it completly disconnected from the main story. No bonuses to the SP, no separated MP mini - campaign or any other thing like that. A horde mode or a PvP mode would be somehow acceptable, even if this game doesn't need any of this things to be good.
If they want to / must have a social part in the game, they can repeat what they did with DA:O and DA 2 and , if they had any plans for a MP mod, try to redirect the funds and attention in the SP.
Let's not forget that some people might not have good internet and they might suffer in the SP game because of this, depending on the way they implement it. That,in my eyes, is unacceptable, expecialy from a company that made a reputation by delivering engaging and good quality single player experiences. The good part is that the DA team is not the same team that worked on ME3, so fingers crossed that they ( and the ME guys ) learned from their past mistakes.
In conclusion, I don't want any MP in Dragon Age because it doesn't need it and it could damage the SP experience not by taking buget from it, but by taking the attention and manpower that could be used in developing the SP world.