Yeah the RNG in this game varies. I saw Rhymstyle do a video with 355 stones and thus far the only thing he got was that SSj3 Goku and Gotenks. He was pissed.
Me I'm only going in with around idk 600-800 stones. Usually on these dual dokkans that's the amount of stones it takes me to get at least 1 of both main guys. If I can't get it then I probably won't summon anymore on those banners out of principle. I've seen some people like Nanogenix and D-Free to amazing with less than 1k stones but seen others who put in 1k and get destroyed. Given my luck hasn't been good I hope it works out.
I posted it pages ago but here are my summons for the past year or so.
UI Goku 2017 Christmas Banner - Got him
Ultimate Gohan - Didn't get him when the banner dropped, got him months later
Kale and Caulifla - Got them
Rage Vegeta and Beerus - Got Beerus, got Rage Vegeta months later
LR Trunks - Didn't get him when the banner dropped, got him months later
SSj3 Broly - Got him
PHY SSj Vegetto - Didn't get him
Universal Survival Banner - Got Bergamo and Fit Buu, got TEQ 17 and Toppo down the line
Kale - Got her
Angel Golden Frieza - Got him
LR Bojack - Didn't get him, got him down the line
TEQ VB - Didn't get him, got him down the line
STR Rose - Didn't get him, got him down the line
Tapion - Got both Tapions
SSj3 Bardock - Didn't get him
LR Vegetto - Got him 3 times within 2 3 +1 rounds
LR Gogeta - Got him 2 times within 2 3 + 1 rounds
LR Goku Black - Didn't get him when the banner dropped, got him down the line
Turles - Didn't get him, summoned a lot
Transforming Goku - Got 1 of him within 2 3 + 1 rounds
Transforming Frieza - Got 1 of him within 2 3 + 1 rounds
LR Goku and Frieza - Didn't get him, summoned a lot
21 and 17 - Got 21, didn't get 17, summoned a lot
LR Cell - Didn't get him
Blugeta - Got him
LR Bardock - Didn't get him
FP SSj4 Goku - Didn't get him
Future Gohan - Didn't get him
Hit - Didn't get him