It's wise to have a simple number in mind when thinking in terms of what real world value all of the in-game virtual items have. TP was relatively easy to ballpark given the approximate exchange rate of 1 cent per TP; simple to compute a final "real world" cost with that exchange. Shards complicate this as it's a second layer, but individually we can boil it back down to one number that we each use to mentally make the exchange.
I did a quick spreadsheet to see for myself and here's what I came up with:
There are six shard packages in the DDO store (don't ask me how we'll all be able to handle such redundancy in the DDO store, but whatever ). Those six packages give a shard to TP exchange range of 5.59 up to 6.5 TP per shard. Given the TP you can purchase during double bonus point sales, the actual cash value of a shard is anywhere from about 3.3 cents to 7.4 cents; buying TP outside of point sales (that is, at "full cost") puts the cash value of a shard between 4.8 cents and 8.6 cents.
As a middle of road example, if you buy the $34.99 TP package on a double points sale, you're spending 0.85 cents per TP; if you turn around and buy the 100 shard package for 595 TP, then you've ended up spending 5.1 cents per shard. That figure works for me, personally, as that's the TP package I've bought a couple times in the past and would likely continue to purchase.
Now, everyone will have varied buying habits (as well as various local currencies - my cash is in US dollars), but if you want to be mindful of what you're actually spending, then it's reasonably accurate and quick to use 5 cents per shard; if you want to err on the side of assigning higher cash value, then pick a slightly higher number for your personal exchange rate - 7 or 8 cents per shard would also be reasonably accurate especially if you tend to buy TP in the smallest quantity packages. But pick a number from about 4 to 8 cents that matches your habit, then keep that one number in mind when deciding what is or isn't a good value for yourself.
So... for example, each gold roll in daily dice would cost me 75 cents(!!!). Each silver roll would cost me 15 cents. The lolth trinket that appeared to sell in the cash market while I watched earlier would have cost me $1.50 at the 30 shard buyout it was listed for.