Each time you buy Conquest for $6, you gain 2 Silvers, and then gain X VP where X is how many Silvers you've gained so far this turn.

In other words, if you have $6Y and Y buys, then you'll get 2+4+...+2Y points, which by a handy summation formula is Y(Y+1)

For the same amount of money, you could buy ~(3/4)Y Provinces, for ~(9/2)Y points.

So we have Y(Y+1) > (9/2)Y, or Y^2>(7/2)Y, Y>7/2.

So you need $24 and 4 buys for Conquest to be worth 20 points, while 3 Provinces are worth 18 points.

Now, that's the simple approach. Factor in that maybe you could buy a Silver first if you had say $21, and also Duchies/Estates with odd amounts, and things get messier. I think at that point, I'd just code something to do it, and never use the results.

Another thing is if you had gained a Silver first(through Rocks or Trader).

Then, if you've gained Z Silvers this turn, you get Y(Y+1)+YZ points, and Y^2>(7/2-Z)Y, Y>7/2-Z

So if you've gained a single Silver this turn, it would be better at $18 or so, a second would make it worth at $12, and a third or fourth and you may as well just buy Conquest instead.

Dang, now I really want to pull off a Trader Conquest combo.....