There is an industry rule of thumb; it takes on average 10% of the original cost of the boat to maintain as new, pay dockage, fuel, etc. This rule is based on moderate use (12-15 days a month), annual haul out/launch and outside storage with the mast in. It includes the cost of most materials but assumes that the general maintenance is done by the owner except for painting the bottom.. There are higher and lower exceptions of course but I have found this to be a good starting point in planning a basic budget for a late model production boat in the 40 foot range.
The single most important piece of equipment to me other than the basics of gps, vhf, depth sounder, speedo is an auto pilot. Second is a good dodger.