Someone did that math once, but I can't find it (might have been on a stream). With targets around 70-80 or more mr, purchasing void staff will do more damage than deathcap (all while being ~1000g cheaper). Having 70+ mr is nearly a certainty with the addition of bulwark. If you aren't fitting a void staff into your builds somewhere, you need to re-evaluate them.
EDIT FOR PROOF : I just did the math out for a level 11 Syndra (maxing R>Q>W>E), who has one of the highest burst ratios in the game. I even padded it in her favor by giving her 5 balls out for her ult (to give her higher ratios). Here are the calculations: Full burst combo: 230(.6) + 160(.7) + 70(.4) + 675(1) Simpler Form: 1135(2.7) 100AP from runes/masteries/dorans or whatever Deathcap ((100 + 120) * 1.25 = 275 AP) Voidstaff (170AP) + 35% pen DC Pre Resistances : 1135 + (2.7 * 275) = 1887.5 VS Pre Resistances : 1135 + (2.7 * 170) = 1594 DC Damage With 80MR : 1887.5 * (100/(100 + 80)) = 1048.61 VS Reduction with 80MR: 80 * .65 = 52 VS Damage with 80MR: 1594 * (100/ (100 + 52)) = 1048.68
Voidstaff wins by a fraction, while being 1000g cheaper. It also scales better per additional AP than deathcap (35% pen vs 25% addition ap), while also scaling against resistances (if the enemies buy more mr). I left out pen from runes and masteries, though if people want I can do it with those too.
TL;DR : EVEN ON HIGH SCALING AP CHAMPIONS, VOID STAFF DOES MORE DAMAGE, SCALES BETTER, AND IS SIGNIFICANTLY CHEAPER THAN DEATHCAP.
So basically, AP mid build = boots+big item->void staff
I like boots+deathcap+void staff, as every bit of AP after that core gets amplified by Void and Deathcap so the gold efficiency is amazing. --- http://www.youtube.com/TimMH3 LoL IGN: I Am Zyra. Aphoristic (12:31): all you do is b****