value of reduced value giveaways

As above, it's 15%.

You can see how much each GA contributes to your CV by searching for yourself on https://www.sgtools.info/real-cv

The CV is 15% until you have given away more than 5 copies of the game, then it starts to drop off as well.

If you'd like more information on this topic, it's covered in detail within the Help section of the site.
Specifically, Help > FAQ > 'What are "No Value", "Reduced Value", and "Full Value" giveaways? How do they affect my overall contributor value?'

There's also this convenient script for adding sgtools links (such as the one provided by WorstShot above) directly into SG user profiles, if you prefer a simpler alternative to ESGST.