The cost depends on whether you book morning/afternoon type trip or one that includes lunch or the cost of entrance to smewhere. We never like more then 3/4 hours and our average works out around £30 each. There is always the option of taking a taxi/coach from the pier and doing your own thing, but remember you must be back onboard the time given as they won't wait. We like San Juan some nice shops so we just do our own thing; Lisbon is an interesting city with old style tram cars but it is very hilly in bits; Vigo we never liked some street markets interesting but we never thought much more of it; Le Harve again some nice shops but closed I think on a Monday, but you can grab an excursion to Paris but it is a long day, depends if you want an eye full !! Seriously if you want to budget any trips bought via the ship are charged to your cabin so you pay at the end of the cruise and worry about it when the credit card bill comes in !!
An alternative to Paris from Le Havre is the beautiful village port of Honfleur, just around the bay about 15 miles.