The only decent reward I notice from chests is the chance of getting purple gear.
Everything else got super nerfed when they corrected the original issue with the chests - that they gave WAY TOO MUCH stuff. It went from one extreme to another.
On a side note your question raises, Plarium doesn't have a good measure of the true value of items. The black market sapphire prices are used by devs when they make updates. But those values are massively out of kilter with the real value players put on them.
If you think about it for a bit, it's quite a hard thing to keep track of. Updates can render previously content obsolete. So even if you start with a great understanding of relative value, those values can be changing all the time. So it's no easy matter to keep accurate.
Next problem is, I suspect people buy sketches and buy troops. But other than that, they buy very few of the other shop items. So you can't use the demand for items to give you hints about how players value the items, with clear info on overpriced items being given by activity in the markets. Or to put that another way, most items have an "illiquid" market, where there isn't enough market action to drive clear pricevalue signals.
I have thought about this quite a bit but I can't see an easy solution.
You can poll users to try and get a feel for how players actually view items, but then you will get feedback that is really difficult to analyse because everyone has different views, plays in different styles, so if only a few players reply to the survey it's very easy to get completely skewed results.
So my final conclusion is that maybe it's better to ditch giving random black market items out. Maybe this is an opportunity to add more content to the game. Chests from the Tower potentially could be used to combine/ create new strong hero armour for instance. Then you solve the problem of giving rewards from existing items with little to no way of knowing how players actually value the rewards, AND you do what the game is crying out for.... add new content and things for people to work towards.
Same would apply to the recent champs quests. Where the value of the items is so poorly understood that incredibly rare 5/6/7 orders give items that often are no better or even worse that what you get from order 1 / 2.