I think it will be very costly in time, labor and expense. Why do you think it won't cost much?

I meant compared to starting from a completely flat ceiling versus starting from a ceiling that is already partially slanted. I'm still expecting it to cost thousands.

"My thoughts are that since the ceiling is already slanted, it shouldn't be too much cost or work to continue that slant to create a vaulted ceiling"

The existing situation is probably already inadequate in terms of ventilation and insulation. Continuing the slanted ceiling on up will require removing the collar ties, which will weaken the roof. The rafters are probably 2x6s and will need to be replaced or sistered with 2x10s or 2x12s. You'll need a structural ridge beam, which means you'll need a new roof.

Thank you for your comment. What if I kept the collar ties and had them exposed like in the picture below? Would that still require a new roof?