The description for the Visa I want, says that I need to have comprehensive Medical & Health insurance while I'm in Thailand. Although they don't ask for any proof, so I take it as more just a recommendation than a requirement.

How much is this likely to cost? (As I assume it's cheaper to use a Thai company instead of an NZ one)

And if I didn't have insurance, how much would something likely cost me? If say, I was in a motorbike accident and needed some stitches + cast for a broken leg or similar?

As if I had medical insurance, I'd probably forget / not be bothered with all the red tape if I actually did get injured / sick, unless it was excessively expensive of course.

If you end up in the Bangkok / Phuket, Pataya, Samui, etc Hospital? you will find that they are far more expensive than back home, no BS.

They prey on tourists and xpats so be aware of that. And they love your credit card and you will not be allowed to leave the country until you have paid in full.

Can we ask which country you are from and what Visa you are applying for ?