I think establishing a good baseline rule for when to spend money on the game is ideal. Nothing wrong with it, just the rule is to remember that there are no guarantees when summoning, ever. Even on banners without off-focus odds, you aren’t guaranteed to get what you want with any amount of orbs. You aren’t even guaranteed to get good orb selection on each round for what you want.
But there are still cases where you can get good bang for your buck at relatively good odds no matter the results. I think it’s all about planning for things going wrong and seeing if it’s still worth it one or two levels of bad luck deep, and only then is it more likely to be money well spent. This Double Special Heroes banner is a good example of a worthwhile banner. My mindset for it went as follows:
Firstly, base odds are 6% and there are no pity breakers. Blue in particular has two amazing seasonal heroes with fantastic fodder that don’t require high merges to be truly useful, Summer Lyn and Summer Laegjarn. I’d also argue that every color except for maybe Red is a good contingency plan if Blue falls through: Green has dancing Reinhardt who is simply ridiculous with any IVs except for an Atk bane (he can even run Sturdy Impact and actually take a hit with defensive IVs), and Summer Wolt provides Brazen Atk/Spd 4 and Chill Spd fodder if you don’t get Reinhardt. Colorless has Halloween Mia for Witchy Wand+ fodder (and isn’t a bad unit herself) which is a crazy-good staff for shutting down specials, and dancing Nephenee as the only bow refresher in the game with great offensive stats, a welcome defensive refresher if she has bad IVs and Spd Tactic 3 fodder if you already have her. Even Red color isn’t all that bad, Bridal Sigrun is a decent sword flier with Air Orders 3 for flier teams and Summer Helbindi with Rally Atk/Def+, R Duel Infantry 3 and Panic Ploy 3 fodder (not great, but not awful either). Plus, all of these units can only pitybreak each other, since there is no off-focus 5* on this banner. This banner is leagues above the other seasonal banners the focus units debuted on simply because of that alone, and base 3% odds to get any banner hero. So to me, this is a fantastic banner and worthy of a bit of spending if you are particularly interested.
Outside of this banner, there are some obvious heroes that scale well into the future because of their Prf weapons and/or skills that are always going to be great. Almost all Legendary and Mythic heroes are excellent for competitive PvP modes. Brave Ike will always be an effective tank because scaling % damage reduction with absolutely no requirements and only a sparse handful of direct counters is insane for most PvE game content and competitive game modes. Heroes with scaling % damage weapons like L!Alm, L!Celica and B!Alm will always chunk any enemy. Bladetome/reverse bladetome weapons will always scale well because buffs aren’t going away anytime soon. Penalty-negating heroes like Idunn, Brunnya and Shannan will always outright negate any penalty no matter how harsh it is–same with buff-negating heroes like Larcei, who also has Repel for scaling damage reduction based on Spd (and if she ever gets powercrept, her skill can go towards Larcei 2.0). These kinds of heroes are prime investments, it’s just a matter of waiting until you have enough orbs to get them. They will always be rerun eventually. Duo Heroes are the only exception; they’re exceptionally powerful and they are all massive powerhouses any player would want to bolster their rosters with, but they have no guaranteed reruns. These are prime investments that might be worth a penny or two every so often to top off your orb count for increased chances.
Outside of simply spending too much money on this game which is obviously iffy as an investment on its own, imo the money-spending is a real concern only if you try to pull for favorites in lieu of strategy; heroes that you don’t really need that can’t contribute with good skills, or pulling on lower-chance banners with off-focus odds. Trying to offset bad summoning luck in these situations with money spending is poisonous for the mind and sets you up for disappointment.