I've seen figures as high as 80% "smokable material" being instantly destroyed by lighter flame. The temperature at which the plant matter burns is higher than that of THC vaporization. Thus, burning plant matter implies THC has already been vaporized and dissipated.

A Bic lighter seems to generate temperatures of around double the point at which the plant starts to burn, thus destroying much THC on contact.

So if you consider that worst case scenario 80% of 10-15% THC content is destroyed, you're left with about 2-3% THC vapors per hit.

Edit - Unheated cannabis isn't absorbed by the human body, hence the use of lighters, ovens, and other heating devices to convert the inert resin, and the common use of 'activating' the cannabis by heating to sub-vaporization temperatures (converts more of the cannabinoids into those enjoyable by humans).