Through trial and error and maxing many SR and higher cards here's what I've learned.
Farm Mission 2-4 till at cap, use the commons (spider woman, sandman) to level your card to 30, or as close as possible. Fuse ALL the uncommon ironmans you get throughout this and only use the rare ironman+ on lvl 30+ boosting. When boosting your final card try to boost using full 10 card boost as soon as possible to get ability upgrades.
If your not to conserned about random ability upgrades use ISO-8s to finish it up. Is recommend saving them for only SSR or higher cards, as it will get very expensive quickly. Calculate each ISO-8 as giving 3 levels.
It helps if you have friends accounts you can farm on when they aren't playing.
I don't have the exact source, but I think boosting cards with the same card type leads to "Ultimate Boost' opportunities. I've been focusing on that lately and it seems to be making a difference. So try boosting a speed card with other speed cards, etc.
When boosting in general, try to do 10 cards at once. There seems to be a cost per card that increases according to the level of the card you are boosting. So to get the most for your money, use as many cards as possible every time you boost since the cost per card will go up after leveling up.
Along with the cost per card idea, there appears to be no difference between the cost of using a common when compared with using a rare or any card for that matter. The boost will be more significant from the upper-level cards, but it still costs the same per card.
Can anyone back me up at all or am I way off on this one?