I slipped up on this with my own gaff. Someone put me straight on it but it was too late at that stage.
Before you do anything, you need to go round there with a straight edge and see if the walls are straight. Maybe theyve got a dodgy plasterer on the job who has messed up.
The builders are asking for a ridiculous amount because they want to put you off the idea. It suits them to have white skirting. When the skirtings in and theres gaps between wall and skirting, they will fill the gaps with filler - so their mistakes are less noticeable.
Furthermore, its a lot more hassle to keep skirting painted as opposed to having it stained.
And lastly, are you doing the flooring or are they? If your getting the house without flooring and are considering putting in wood or laminate flooring, then this should be done before the skirting goes in. Timber flooring contracts and expands so a gap must be left - this gap will not be seen if its hidden by the skirting - and this makes a far better job of it.
In my case, I wasnt up to speed on this and will have to rip out all the damn skirting, do the flooring, then replace with new stained skirting.
Think how much extra hassle this will be - and you will be well motivated to tackle the builder on this.