Just got an email about this exciting new way to spend your Miles (not), which states upfront the direct ratio: 5,000 miles = 40 USD. Meanwhile, reward flights are also being rebranded as 'classic rewards'(!)
"It’s now even easier to spend your Skywards Miles with the launch of Cash+Miles, which allows you to instantly bring down the cost of any ticket you book.
If you don’t have enough Miles for a reward flight – now known as a Classic Reward – you can pay for your ticket using a combination of cash and Skywards Miles*. All you need to do is choose the number of Miles you’d like to put towards your flight, from as little as 2,000. If you convert 5,000 Skywards Miles, your fare will cost USD 40 less. And you’ll even earn Miles on the part that you pay for using cash"