They are all pricey if you are getting the right kind. Solarmeter 6.5 or 6.5R. They run $200-250
Honestly unless your getting sun shinning into that cage your not getting UVB for him. Then there is the fact that it is a solid roof screened in porch and he is also in a screen cage. So your getting a reduction in UVI level through each layer of screen and the fact that the roof is there. Depending on time of day and if their is actually a position where the cage gets full sun your looking at lower UVI levels. UVI levels are higher in the day when the sun is at its peak in the sky. As it sets and rises they are lower.
So essentially you may want to just consider providing a T5HO fixture and a 5.0 bulb at the correct distance.