Most SRs aren't useful and only exist to take up a slot in your inventory. I've been slowly making my way through my collection, clearling out the lower point SRs. Super expensive, though. I went from 30 million gold down to under 5 million. Too many junk SRs.
if you're willing to take more time to lvl cards to max, it's most efficient to use Lvl 1 Ns than lvled up Ns (for that matter low Lvl > high Lvl and higher rarity > lower rarity, priority to the latter). so I sell all my Ns above Lvl 1 and use exclusively Lvl 1 N and higher rarity to feed (for that matter, when I finish Lvling Ns to 40, I sell, not feed). I started around 30m before 4.0 and still floating around 20m (in part because of the Token event just now and using 13 gems)
5m is starting to get really low in G, especially if we're expecting a Challenge event on the 31st
math for those interested
Lvl 1 gives 120 exp at the cost of 1000 G or sells for 1400 G so if we're feeding exclusively with Lvl 1s, to Lvl a N from 1 to 40, you 8000 exp / 120 = 67 Ns. at 1000 G per card and opportunity cost of 1400 G, it's effectively 2400 G each so costs
160,800 G
Lvl 10 gives 384 exp at the cost of 2900 G or sells for 5180 G. that's 8000/384 = 21 Ns. 21*(2900+5180) =
169,680 G
so feeding with Lvl 10s costs more
and to show that my math works the way it should with opportunity cost, I'll sell the amount needed to lvl for one group and feed the other group and see how much G I have in the end
Feed with Lvl 1s and Sell Lvl 10s
costs 67,000 G and gain 108,780 G. surplus of 41,780 G
Feed with Lvl 10s and Sell Lvl 1s
costs 60,900 G and gain 93,800 G. surplus of 32,900 G
first option nets us 8,880 G more, like in previous method
this is because the increase in G gained from selling grows faster than the exp/G ratio as the card lvls. this is also true for Lvl 1 R to Lvl 10 R but R are just much more cost effective than N