Well, there is no strict cap as far as I know, but cost reduction in general is calculated in strange ways.
Cost reduction cp works as you would expect it, it reduces costs exactly by the number it says. Same goes for jewelery enchantments.
If you stack cp and enchantments. The cp are applied first and then the jewelery value is substracted from the number that you got after applying cp.

If you use any other way to reduce skill costs, the reduction will be lower than stated in the tooltip. You will only recieve about 86% if the stated value. So a passive that says 1% cost reduction in the tooltip will only reduce costs by 0.86%, a passive that states 30% cost reduction will only lead to 25.8% lower costs. The same applies the other way round, the cost increase by the Molag Kena set or for multiple streaks in a short time will also be only 86% of the tooltip value.

If you use both, cp and other cost reductions, they stack multiplcative (and other cost reductions are still reduced by 86%).

If you stack enchantments with other cost reductions, the enchantments will applied first and the other cost reduction will be calculated from the reduced value (and still be lowered to 86% of the tooltip value).

If you stack all three, it gets crazy:
=(Base Skill cost*0,86*(1-other cost reduction)+Base Skill costs*0,14)*(1-cp reduction)-jewelery reduction*(1-other cost reduction*0,86)