We booked all our excursions third party. We did do a dog cart ride, it was short and part of a larger excursion. We didn't do anything involving planes or helicopters. We had an AMAZING time and I wouldn't change anything. Here's what our family of three (two adults and an 8 year old) spent. (This does not include the things we did in Vancouver, which were also fantastic.)
Skagway: Yukon bus and rail with dogcart ride via Chilkoot Charters - $600 (bus up to the Yukon, lunch, dog cart ride, White Pass rail back down)
Juneau: Car rental $60, explored the Mendenhall glacier on our own (I think it was maybe $5/person for the park entry), came back and had lunch on board the ship, then private whale watching tour (that we split with another family of three). Our portion was $367 for the whale watching for the three of us
Ketchikan: Explored and walked around on our own, lunch on board the ship and then an afternoon excursion of sea kayaking which was $250
So our total for a family of three was around $1300, which I think is reasonable. It sounds like your initial budget was around $400/port so $1200 total? So we had only three of us, but I think you can come in pretty close if you maybe budget more for some ports in others. For example, our kayaking excursion was only $250 for the three of us, so we spent less in that port than your budget would be (it was $89/adults and $59/kids) and the kayaking ended up being my daughter's favorite thing. We kayaked right under the bow of the Wonder out into the channel and had an amazing view! It was really incredible!
For our bus/rail/dog thing, it was all day and included lunch. So for an all day tour for $600 for the three of us, I think it was really quite a good price. Chilkoot was great and you'll see a lot of others raving about them here on the DIS. That was my husband's favorite excursion.
So for us, basically, we chose to have one really expensive day (Skagway), one mid-priced (Juneau) and one less expensive day (Ketchikan). It made sense to us to do it that way. So you could do one dog sled and have that be your thing and be more low key/ cheaper for the rest of your ports.
If you want more details about any of these excursions to get ideas, my trip report is in my signature. There's also a HUGE thread of Alaska excursion reviews that's pretty helpful. You should be able to search and find it. I think it's something like "Post your Alaska excursion reviews here."