The number of crap mods you take to L12 is huge. It's not a ton per mod, but you get SO MANY crap mods that the number of credits you spend per good mod you end up with is in the same ballpark.
If you're someone who still struggles with credits, then sure, factor that in bc while I haven't done a rigorous calc, it appears likely that it's somewhat cheaper.
But even when you take mods being sold for ship credits off the table (which I have to do at times, for sure) only counting the regular credit mods for sale, you still end up with more good candidates than you do farming.
Elsewhere I've mentioned that targeted use of mod challenges when way behind on a specific mod set is absolutely a fair short-to-medium term strategy.
But if you're constantly picking up the good mods available to you in the store, you'll get a good variety. And 500 is a large enough inventory that you can have Off Sets and Def sets, etc. waiting around for the next toon. You can also upgrade a crap toon with great mods to make sure you don't accidentally sell them or something. For instance, once all your good tanks are modded in a way you're happy with, try modding Nebit as good Def mods come in. Then when the Era is over, you move Nebit's mods to your new tank and throw junk on Nebit again before you slowly replace the junk mods, one at a time, as you get new Def mods from the store.
This is a great way to construct useful sets to have waiting for your new toons, and maybe even get some benefit out of them while the Era is ongoing -- at least near the end of the era. For a while you'll have mismatched sets, but it's Nebit, so who cares?
Anyway, just try going full out on the mod store. My credit supply is so large and my income so high that I struggle to keep it from growing. If you don't have a bunch of R9s for the newest Assault Battles that drop 20M credits in a day, then your income might be different and your needs different.
But I am the designated help person in my guild and ALL of my new guildies have benefited from moving to store-heavy acquisition. Most of these players are 12.0-12.9M when they arrive in the guild, so not players who have everything maxed out, but still largely endgame folks.
I don't worry about emphasizing the store too much, bc if people are already used to the mod challenges, I don't have to tell them the benefits of that strategy. it's the store that's under-utilized. So I can talk up the store and not worry that they'll forget they always have the short term option of the challenges if they need a particular mod set.
What about you? Have you tried going hard on mod farming from the store?