Jim,
The Alaska port board at cc is a good source of info for cutting costs. Anything outside of the towns is gonna be more expensive since there are no roads to speak of, hence helicopters & planes.

Looking at the possible excursions I'm guessing that these would fall into your price range:

Ketchikan:
Great Alaskan Lumberjack Show
Totem & Town Tour
Ketchikan- Alaska's First City Highlights
Ketchikan by Horse Drawn Trolley

Juneau:
Historic Juneau Gold Mine Tour
Mendenhall Glacier & Salmon Hatchery
Mendenhall Glacier & Juneau Highlights
Mount Roberts Tramway
Healer's Guide to the Alaska Rainforest (?)

You can also take the MGT to Mendenhall Glacier for about $15/20 R/T. And the Mt Roberts Tramway can be done on your own. I think there are hiking trails around both areas.
Whale Watching would be around 100-125 I think.

Sitka:
Russian America, Raptor Center & Russian Dancers
Historic Sitka & Nature Walk

Skagway:
Historic Skagway - Gardens, Tea,& Klondike Experience
Skagway by Streetcar
Ghosts & Goodtime Girls Historic Walking Tour

The White Pass Train will probably be about $90 & up depending on which itinerary you take.

There are some hiking and/or bike tours but I have no idea what they might cost.