I would expect way less of them to be playable. Probably only the 8 planned originally. This wouldn't inflate costs much at all. Not all of them even need to have a lot of dialogue, just the minimum to make them unique.
I'm not even that confident there will be any more companions beside these first 8 playable as "Origins". In fact, being a work in progress, I'm not even 100% confident we'll get all the eight ones datamined so far. So there's that.
And I will openly challenge the notion of it having a low impact on the budget (both in terms of money and manpower) that couldn't be better focused elsewhere, being it "more companions" or "different features" altoogether.
I think it would also be nice to have the ability to create mod companions (like in Skyrim.) Anyone know if this type of modding can be done in Larian games ?
Flexibility of the modding tools aside I'd expect it to be possible.
But the challenge when it comes to modern modding is that the entry level raised considerably compared to the age of Infinity Engine games. You can't have just a standard character model with a copy-pasted Luis Rojo portrait picture and few lines of scripting.
Modding that wants to merge seamlessly with the game will have to account for animations, voice acting (this used to be out of the question, but in recent times many high profile modders started hiring amateur/beginner voice actors for their creations) and all that jazz, almost necessary to make the custom content blend naturally.
