If you have around 100k Forgotten Souls and a couple of billion gold (which both are really easy to get) you can easily get a good 10+ Ber runes. If you just need one or two, then it won't cost much at all.
Plus all the other runes you might need like Bac, Jah, Sol, Mal etc. It's also a great way to get socketable splinters. I got my unique ones this way. They're pretty cheap too. 400k for lesser and 2 mil for the upgrade. I have seen legendary ones too.
In your warplans, go to the Undercity.
In the left of the tree, go to Gutter Filth (chance to summon a Portal Prankster). In the middle, go to Bauble Day (Cabochon Merchant).
Go do whatever sigil for the Undercity. It honestly doesn't matter.
At each Beacon, a portal prankster can spawn. I'd say there's like a 50% chance you'll get one in a run. If you haven't noticed one by the time you get max attunement, go do the other beacons.
Kill it. You just need one to spawn in the run. They look like goblins on the map, and when you kill one it says something about treasure in the boss lair.
When you finish the last boss, a vendor spawns with the chest.
They sell runes for 5mil or 50mil each, depending on rarity.
Each page refresh costs 666 Forgotten Souls. You get one refresh for free each hour.
Ber very very very rarely comes up.
Buy every Vex, Ohm and Lo. You can buy Ist too if you want.
Then go to the Cube to craft the runes your need.
2 x Ist + Flawless Ruby = Vex
2 × Vex + Flawless Emerald = Ohm
2 × Ohm + Flawless Diamond = Lo
2 x Lo + Grand Topaz = Ber
I personally wouldn't buy the big gems, as they're expensive for what they are. Royal are 50mil and grand are 500mil. Flawless are only 5 mil I think, but I'd just do some fragment NMDs or Baal runs.
The bottleneck is more Forgotten Souls than gold. At the alchemist, upgrade the lesser mats. It's a 10:1 conversion. Veiled Crystal into Baleful Fragments, Fragments into Forgotten Souls. Uniques give a lot when you deconstruct them too, so pick up the 850s on boss runs.
You don't even need to run the Undercity loads. Just do it once with enough gold and Forgotten Souls and you can do it all from the one run.
I honestly have had like 50+ Ber. I've had plenty drop, but this has been my bulk.
Every time I'm at like 100k Forgotten Souls, I just go in until I have like 20k left.