Its interesting these offers. I have had them a number of times over the last 2 or so years and some patterns emerge:
- SGD225 SINBNE (which I took) - Y Class was oversold
- AUD215 BNESIN (which I took) - Y Class was oversold
- MYR1200 (ca. 350USD) KULDXB (didn't take but should have) - Y class was oversold
- THB25000 (ca. USD850) BKKDXB (didnt take) - entire aircraft was about 50% LF
- AED3100 (ca. USD840) DXBLGW (didnt take) - Y class was oversold
- AED2900 (ca. USD790) DXBBKK (didnt take) - busy but not full

The only time I get a really good offer (read cheap, in my opinion) was when the aircraft was oversold. It seems when Y or J is empty, they are just trying to get additional revenue so they don't heavily discount the upgrade price, conversely when they know they are going to have upgrades required, they offer them out cheaply just to get some revenue rather than giving it away for free.

Its all a gamble, it really depends on what you see as a good value proposition, as a member above describes.

Enjoy your flights!