It would depend who is offering the land tour. Saving thousands of dollars indicates to me that either very little is included, or the caliber of the hotel(s) and transportation is much lower. There are several good tour operators in Alaska, and I wouldn't hesitate to use one for a one-day shore excursion. I also wouldn't hesitate to book with certain tour operators for a multi-day land tour, but others - well, let's just say there's a lot of businesses offering their services who are less than ideal, and it's hard to know just who's on the up and up. Remember - you get what you pay for.
If you do want to look into booking a land tour separate from the cruise line, please do see a good travel agent in your area, especially one who is familiar with or specializes in Alaska, as they will know the best tour operators to work with.