This is more complicated. We'll boil it down to (( average # of days in a year the pig can produce on ) / 112) * ( average # of truffles produced ) * 1.2 ( Gatherer ) * 1491G (Truffle oil sold w/ Artisan)
First off, we need to calculate the number of days in a year the pig can produce.
Spring/Fall: Out of 56 days, 6 are guaranteed sunny (1st/festivals). The remaining 50 have an 18.3% chance to be rainy, so on average, 46.85 spring/fall days can be produced on.
Summer: Out of 28 days, 3 are guaranteed sunny and 2 are guaranteed stormy. The remaining 23 days have an average 16.3% chance (rounded up) to be rainy or stormy, so on average, 22.25 days in summer can be produced on
Thus, 69.1/112 days can be produced on.
The wiki lists the average # of Truffles to be 3. I'll round that down to 2.98 to account for both the tiny fail chance as well as the fact that a pig cannot produce infinite truffles (I arbitrarily set the upper limit of possible truffles to 9 truffles a day per pig)
Using these variables in the formula, we get 3289.54005, which I decided to just round down again