"Is the cost of preventing climate change 50 times higher than the cost of adapting to it?"
It might be, but Topher's argument doesn't establish that, and one of his references directly contradicts it.
He assumes that the cost of climate change is summed up solely in the damage that a 3°C increase in temperature would cause. He assumes that "climate change will cost us roughly 1.5% of global GDP if we simply adapt to it as required". For this figure, Topher cites (Stern, 2006, p. vi).
However, the report actually says on p. vi:
the overall costs and risks of climate change will be equivalent to losing at least
5% of global GDP each year, now and forever. If a wider range of risks and impacts
is taken into account, the estimates of damage could rise to 20% of GDP or more.
That report also says on the same page:
In contrast, the costs of action – reducing greenhouse gas emissions to avoid the
worst impacts of climate change – can be limited to around 1% of global GDP each
The report that he uses to establish the 1.5% of GDP actually says 5-20% of GDP. It also argues that the cost of avoiding the worst impacts can be limited to under 1% of global GDP.
Topher picks the 1.5% figure from page ix, where the Stern report says (emphasis mine):
Most formal modelling in the past has used as a starting point a scenario of 2-3°C warming. In this temperature range, the cost of climate change could be equivalent to a permanent loss of around 0-3% [...]
However, those earlier models were too optimistic about warming. [And goes on to argue for their 5-10+% cost estimate.]
The Stern report doesn't support a 1.5% cost estimate. It establishes itself as counter to that position, yet Topher cites the report as supporting this estimate.
Topher has incorrectly interpreted the Stern report and cherry-picked from it.
If the Stern report is reliable, then Topher Field is incorrect.
If the Stern report is not reliable, Topher Field no longer has a basis for the "cost of adapting to climate change" portion of the ratio.
The above should be sufficient to show Topher has cherry-picked the numbers he's using, but I'll give one more example.
He assumes the cost of preventing climate change can be estimated by looking at the cost of Australia's carbon tax system. (See minutes 0-5 of the video.)
Even if he is correct in his analysis of the effectiveness of Australia's carbon tax, there are many other methods of mitigating climate change, and he doesn't consider those.