There isn't such a drop calculator but I can give you an estimate on rattlecage (I found two myself). First we look which monsters actually drop the most gothic plates, we find out Urdars, Cliff Lurkers and Dark Familiars drop only 25% the amount of gothic plates we should expect. If you encounter those monsters in your mapseed don't bother. Then we move up to some actual chance:
Act4-HtH is responsible for Gothic Plates. Due to a "bug" all act4 monsters also drop partially from "Act4-Caster". I believe your chance of finding a Gothic Plate would be combining the odds of Act4 HtH, Act4 Caster and Act 4 Misc which results in a plain value of 52, them we should apply the good drops aswell (30 possiblilities with low odds, lets estimate 10%) we get 55. Now we have always a 10% nodrop at p5+ so our 55 increases to 61. Now if we apply 0,33% uniques with 200 mf% your rattle cage has a drop of 1 in 18518 with 5 players in the game.
Buttt. Is Rattlecage that good? Imo if you want to hurt things bad you need to switch your lovely pike with a Partizan or a Bec de Corbin and use a rare ornate with FHR, high def and dex and if possible also 4x lr (fhr matters when going in ww).
I play a CE necro and I accept the fact I can't beat Lord de Seis and Diablo. This fact doesn't hurt my playstile as I REALLY DONT WANT my nec to get ABOVE LEVEL 74. A ce necro is by far the best option passing xp down to other charcters. You have to accept the wait with Corpse explosion and imo run p5 (which results in the same amount of items anyway!). If you want to skill something skill Fire Golem or Golem Mastery for Cow Level. A CE necro shines with 200 mf in cow level (done in less than 4 min. P8 is overkill and slows you down unless you really want to pass as much xp possible.
I was thinking of using the Rattlecage for a gold find barb, with the CB helping to kill champion packs with dual patriarchs. I figured with how often WW hits, the CB would trigger so often that the low weapon damage would hardly matter. I've been saving gold find rares and gambled a pair of chance guards and a couple tarnhelms, so I have something at least decent for every other gear slot except for an amulet, and I was also thinking that farming urdar champion packs would be a really good way to get to a high level for gambling amulets, so gambling Rattlecage instead of amulets now might help me gamble more amulets later. I gambled Goblin Toe awhile back, so I could use those for CB, but I have some pretty good rares with gold find I'd much rather use instead.
And I'm not sure how many act 4 full clears it takes to kill 18518 monsters but considering it's kind of a pain to do with 200 MF (either on sorc or necro) compared to how fast a barb just tears through everything except CS, I'm thinking I might want to go ahead and gamble it after all. Just selling the drops from one act 4 clear nets enough gold that the barb's faster killing speed probably trumps the MF, though I could also do pretty well with a lesser amount of MF on the barb, say 100. At the very least, I can easily use my 48% tarnhelm over wormskull, especially since my current pike has 5% life leech so I can definitely afford to give up a little from another slot.
The other thing to consider is that I'm going to be doing a bunch more cow king runs regardless, since I'm still looking for a lot of stuff he can drop (including a good rare ornate plate like you mention), so I can use the gold I make doing that to gamble Rattlecage. And if I'm going to wait to gamble mass amulets until I have a higher level character (highest currently is about two-thirds of the way to 86), there's not that much else to spend gold on right now (not counting sashes and boots since I always grab those when they show up on the gamble screen, but they are so cheap it hardly makes a difference).
I'm certainly keeping an eye out for any good rare polearms that might drop, especially once I get my gold find barb farming urdar champions since I think they have a pretty decent chance to drop rares (though a Grim Scythe with -40% requirements from cow king could work too). But I'm still using a relatively mediocre pike since I've had pretty bad luck with them so in terms of difficulty vs benefit I think it makes sense to keep trying for a better pike from cow king for now.
And yeah CE tears through cows, MF gear or not, and there's definitely no need to put points in anything else for cow runs but that's pretty much the ideal environment for CE and I was hoping to make my necro kill faster elsewhere, especially since it seems like there are plenty of skill points to go around. What's the logic behind putting points in fire golem though? I don't see the reason to use it over blood golem.
