Jayne, they don't include tips in the price, but they problay should since it is almost mandatory to tip.
It IS in all the brochures, so if you had read them you should know about them( in fact I believe you do, since you've inquired) .
it will be 20$ extra total( for the two of you) per day. So a 20 day cruise is 400 dollars . Considering how strong your pound is in converting to American thats not so bad.
Don't just not tip, they will spit in your food( just kidding, no, may be not)