Well, it all depends on your preference for appearance and utility. Do you want to simply use the normal construction grade rafters and blocking? Do you want to look up and see the bottom and raw edges of the roof sheathing above the rafters? Do you care about seeing the roofing nails penetrate the roof sheathing? How will you route electrical power, lighting, signal and HVAC? When you omit the ceiling joists how will you offset the outward thrust of the rafter bottoms on their supporting walls? And how will you support the ridge beam which supports the upper ends of the rafter? If none of these things matter, then simply deduct the cost of hanging the ceiling gypboard and proceed to the bank with your savings. On the other hand, if you have to add materials and labor for any or all of these things, add by a measure of 1X-5X, depending on complexity.

What happens with the insulation?

The devil is in the detail . . . and so is the cost. Figure out how exactly you want it constructed and contact local builder to give a guesstimate. Prices in different geography areas can vary greatly. Any answer you get here will potentially mislead you.

Exterior insulation for a roof with a pitched interior ceiling must go: 1) into the rafter space above the ceiling; 2) above the roof sheathing; and/or 3) a combination of both, depending on the required or desired rating. Ventilation under the sheathing must also be accommodated.

where you putting the insulation? you will have to meet code's R-value for your climate for this area. give some thought as to how to accomplish that.

best of luck.

If the OP will have an insulated and finished sloped cathedral ceiling for both options, then the only difference in the two options will be that one uses exposed rafter ties between the wall top plates and the other uses a ridge beam and post supports instead.

If that is the case, I suspect the rafter ties will be cheaper unless they will be doubled up and spaced every 4 ft and wrapped with wood or drywall and finished. It all depends on the labor involved and for that a complete design must be proposed for each option so a contractor can price them.

Thanks all. I'm asking because our addition roof was started with rafters with a ridge beam and post but the designer had intended scissor trusses because of a semi circle window to be placed in the gable. We love the height of the rafters and were wondering if anything could be done to keep that height but still allow for the window. Wasn't sure if doubled up ties could be used instead of the post support or what the additional cost of finishing those ties would be (preferably with a wood look)

If the ridge beam is already in place that would be the cheaper option.

Can't you do a non vented roof,a mix of OCF under and polyiso above roof deck?

All of the options described above are available to you but estimating the cost of them on this forum with so little information will not be helpful. You should hire a structural engineer to design the least expensive structure given the current status of the project.