I believe it used to be base + 0.24*AP, but I don't know if it's changed since 2.0. I haven't been able to make myself level my rogue past 62, so haven't got anything to check that against.
On that note, though, it was my understanding that Rip had the same base+0.24*AP formula at lvl60. I have played my druid a great deal, and with ~2300ap a mangled rip (+30% dmg) does ~3k damage over 12seconds. At ( 1092 + X*AP ) * 1.3 * 1.1, X ends up being around 0.44. Unless I'm missing something, that's a large change. Hence my concern that the Rupture formula has changed on me when I wasn't looking.
Combo points invested definitely affect damage per tic. Tested on the Unliving Residents outside Kara about 15 seconds ago. My rupture ticks for 96-97 with one combo point invested and 171-172 with five invested. I have 1542 AP at the moment.
I'll go ahead and see if it stays consistent while I'm waiting outside.
Looks to me like the base damage of Rip is not what we've been postulating. Based on that data set, we see that increasing AP by 592 increases the damage of 5pt Rip by ~162. This means each AP is increasing the damage of a 5pt rupture by roughly 162/592 = .274 damage; but that's after the 10% bonus, so it's increasing the base damage by .249 per AP. Now, that looks a whole lot like it's supposed to be .25, so lets posit for the moment that the damage of a 5 point Rip is supposed to be X + .25 AP. Then 1.1 * (X + .25 * 2303) = 2433... solving for X yields ~1636.
Now, curiously enough, TT Rip is 1092, and 1.5 times 1092 is 1638 - very close to our number. So it looks to me like the base Rip damage is 50% higher than listed, and the actual Rip damage output is given by 1636 + .25*AP.
Lets check this for 1711 AP: 1638 + 1711/4 = 2066 - throw in the 10% bonus, and we have 2272 damage, which would indeed tick for 378 and 379. So it's at least plausible.
Unfortunately, the lower combo point numbers don't work out so elegantly, so this still isn't a full picture of what's going on; but for 5 combo points, 1.5*1092 + .25*AP seems accurate.
One thing that many people seem to be missing is the fact that Rip lasts 12 seconds (6 ticks) no matter how many combo points, Rupture ticks longer with more combo points (4, 5, 6, 7 or 8 ticks). Obviously Rip ticks will increase substantially with more combo points when compared to Rupture which relies more on the increased number of ticks.