There's the boat purchase price and the cost of ownership price. Going up every 3 ft roughly doubles your ownership costs. So a 33 ft boat is 2X more costly to own than a 30ft boat. Storage fees, taxes, bottom paint, sails, moorage/dock fees are more expensive and recurring costs.
First question when looking for dockage for the night, winter storage, taxes, etc....."How Big is Your Boat?"
I could not disagree more. While the cost of boat ownership goes up as the boat's displacement increases, and to a lesser extent as a boat gets longer, In my experience the price of boat ownership does not even begin to double when you at 10-15% to the length of a boat. In reality, cost for sails, bottom painting, hardware, and so on are more directly related to displacement. Insurance is solely related to age and market value. Storage and dockage is somewhat dependent on length with marinas ever increasingly including bowsprits and davits in the length, but lengths are typically bracketed with the same price for a given bracket. But dockage is also a portion of owning a boat, maybe 20-30%, so increasing the length by 20% only adds perhaps 6% to owning a boat. It does not double the cost.
If there are significant factors which controls the cost of ownership, it is the condition of the boat when you bought her, the original build quality, original design quality, and your ability to do your own maintenence and keep the boat in solid condition during the period she is yours.
Buying a beat up boat means there will probably be a lot that needs replacement that you and the surveyor cannot see. Buying a boat with mediocre to poor build quality means that it will wear out sooner and could develop potentially major repair costs that a better built boat might avoid. Buying a boat that has been religiously maintained means there is less to replace and upgrade over time. Being able to do your own maintenance, and actually doing it religiously, is a reward unto itself, but also means a familiarity with the details of your boat so that things get repaired and maintained before they fail and lead to bigger problems and expenses.