Okay, so. Supply contracts revisited. I'm pretty sure I understand the new system; it's actually much simpler than I thought. The basic premise now is to contract the cheapest supplies in terms of their per-unit price.
So, stuff like electric power that costs $60,000 per unit and especially FMU which costs like $2.6M per unit are always a horrible thing to contract and should be bought at 100 quality. But, cheap stuff like high tech services (~$250/unit) and services (~$290/unit) are usually good to contract. It depends. You'll have to look at each corporation and take into consideration what it consumes and the relative prices of each.
If contracts confuse you, there is another way to save a bit of money easily. Set your corporations to order supplies at 160Q (190 for state/country controlled). You can buy factory maintenance units in large volume at 100Q with your enterprise or country. Then, on the world trade page, go to "enterprise (or country) trading with your own corporations" and sell 60 months worth of FMU to all of your corps. This allows you to save on FMU costs in all of your corporations since FMU has an almost non-existent impact on final product quality. You can also do this with electric power, and possibly other expensive things. The reason I don't like this is because of the micromanagement involved. You have to order stuff regularly with your enterprise/country and then remember to fill up your corporations every 45 or so game months (if you let it get too low the game will auto-order 160Q stuff) which is a pain in the ass. I'm more of a "fire and forget" type of person so I go for the contracts.
Using supply contracts from your own corporations also has the added effect of boosting the supplying corporation's income a little bit since while there is a cap on world market selling price (296% AFAIK), contracted products receive $$$ according to their quality (so 333Q gets 333%, and truly-publics supplying 370Q get 370%).
None of this is really earth-shattering, and if you're not an obsessive-compulsive like me who likes to fine tune everything, you won't automatically fail at simcountry for not doing it... but every bit of game money helps.