Have you priced out teak lumber? My closest guess is a 6' wide by 8' long platform would run close to $2000. Each plank 3/4" thick, by 5 1/2" wide and 6' long would run about $120 including shipping. You would end up with 16 of these for an 8 foot long platform: If you went with 1/2" thick (which I would advise against) you would save about $300.
Then the cost of the platform.
I also would wonder about the advisability of another 150 lbs on the roof in rough weather. I have run the 25 in some fairly rough weather in both BC and AK--and I was glad that we only had 30 Lb of dinghy up on top. The 150 lbs will affect the stability of the boat.