For tours of a specific sight, such as the Doge's Palace, your least expensive option is virtually always going to be found on the web site of the attraction itself, assuming it offers tours. I know the Secret Itineraries Tour at the Doge's Palace is very popular on this forum.
Private companies will nearly always charge substantially more than the attraction being toured. YMMV as to whether the extra cost is worth it.
Commercial tours can be life savers when there's something you really want to see but neglected to book early enough. That probably includes the Doge's Palace. If you arrive in Venice without tickets for San Marco, you can stand in the ticket line; it will probably look terrible, but it moved quite fast in September 2022.
A key thing to know about Venice is that the very useful (sometimes essential) vaporetto tickets cost 9.50 euros per ride. That can build up to an exorbitant figure. It's better to give some thought to how many trips you know you will take (along the Grand Canal at least once, Murano? Burano? etc.) and consider whether one of the passes will save you money:
24 hours: 25 euros
48 hours: 35 euros
72 hours: 45 euros
168 hours (1 week): 65 euros.