CARAVAN? You peoples is crazy.
If this was turn 3, and you opened Silver/salvager your deck is:
CCCCC CCEEE Ss - original avg hand value = 5 * 9/12 = 3.75
trash an estate for a lab, then the chance of having the lab in hand is 5/12. avg treasure value of other cards is 9/11.
avg hand value (estate->lab) = 5/12 * ( 6 * 9 / 11) + (7/12) * ( 5 * 9 / 11 ) = 4.432
trash a silver, gain a gold, instead you've now got an average value of 10/12:
avg hand value (silver->gold) = 5 * 10 / 12 = 4.167
trash a copper, gain a caravan. Odds of a 6 card hand is the same as with lab, but delayed, so it's crappier. But so is your money density, making this the worst option:
avg hand value (copper->van) = 5/12 * ( 6 * 8 / 11) + (7/12) * (5 * 8 / 11) = 3.940
Lab looks like a much better buy, gold passable, caravan silly.
Since this is turn 6, I'll assume instead you've just been pumping silver into your deck, so your starting deck is:
CCCCC CCEEE SSSSs - avg hand value = 5 * 15/15 = 5
avg hand value (estate->lab) = 5/15 * ( 6 * 15/14) + (10/15) * ( 5 * 15/14) = 5.7142
avg hand value (silver->gold) = 5 * 16/15 = 5.3333
avg hand value (copper->van) = 5/15 * ( 6 * 14/14) + (10/15) * ( 5 * 14/14) = 5.3333
or maybe, you've trashed an estate, but still only got silver:
CCCCC CCEE SSSSs - avg hand value = 5 * 15/14 = 5.357
avg hand value (estate->lab) = 5/14 * ( 6 * 15/13) + (10/14) * ( 5 * 15/13) = 6.593
avg hand value (silver->gold) = 5 * 16/14 = 5.714
avg hand value (copper->van) = 5/14 * ( 6 * 14/13) + (10/14) * ( 5 * 14/13) = 6.153
This all assumes for simplicity that salvager doesn't help your hand value. That's not true, so the figures won't be exact. But doing the math of estate collisions is too much for right now, and each of the salvager turns involves the same cascading math problem, but salvager's value is going to be increased by having a wider variety (bigger hand) of things to trash.
In short: trash estate, get a lab. There's corner cases, but you won't go wrong with lab.