I assume you already have a Quad LNB since you are running an additional standard digibox so there is the cost of 1 new cable for connecting to the spare LNB output.
Subscriptions....
If you have the SKY+ fee waived due to having two or more SKY premium channels then the mirror subscription (once activated) will also have the SKY+ fee waived.
So basically there is no change in the total price you are currently paying when changing the standard digibox to a SKY+.
I am not 100% sure if the same applies to those who have a channel package that requires the £10 SKY+ sub to be paid, I would assume the second SKY+ also requires an additional £10 paid as well.