No card that simply gives +$x changes the value of Laboratory draw like that.
I think I found secret chamber's +$1 per dead card to be useful in more games than Coppersmith was. (Vault and Storeroom are even nicer obviously, but they do more than simply give +$x)
For example the Hoard / Mill synergy. I don't know if gaining a Gold while buying a Mill is supposed to be additive or multiplicative. I would call it a synergy because Hoard increases how "efficient" Mill is for you, because the Mill now comes with a Gold when I normally get the Mil for other purposes.
Yeah true, I just wanted to give an example of a lighter synergy -- so we can see the difference from a full "combo". One might argue that, hey, Hoard is just doing what Hoard says it does, what's so special about combining it with Mill?
The reason I consider it a synergy is that with this combination (in the ordinary, non-"combo" sense) you can buy Hoard and VP cards sooner than you otherwise would, if Hoard and base VP cards were the only ones available to you.
With only base VP, you typically buy Hoard after a Gold or two, once you're ready to grab your first Province on the next cycle, right?
With Mill around, you can buy Hoard with your
first $6, get a Mill+Gold or two on the following cycles, and then be ready for your first $8 Province+Gold soon after that. So Hoard+Mill are accelerating the early-green strategy, which I see as a synergy. Harem and Nobles are of course even nicer and I sometimes go double Hoard with those.