I would imagine the purple IO set is best by far.
As for affordable, I don't know how good the +resist aura or +defense aura is... so I would probably take blood mandate, or otherwise max out accuracy and damage. Endurance reduction might also be slightly helpful (it effects pets' power costs in addition to the cost of the summoning power itself) Generally, you take the pet damage sets that have set bonuses you desire.
You may also want to consider +build up from the purple set even if you won't be taking the rest of it. Procs in general can be useful, but they only proc with the associated attack type (knockback smash damage procs would only proc from knockbacks, and so on). +recharge procs are worthless to pets, they will simply be immune to them when they trigger.
I know that it's good to buff the resist shields for demons, but don't bother with heals or any other effects for them. Demonlings don't need anything but accuracy and damage. Demon princes... I don't know, honestly. You had better ask someone else on that one. Holds might be worth it, or might not. I also am not sure if ghost widow's +psionic damage proc is useful for them.
I don't want to give bad advice, so you may want to check and make sure with others that this is correct and accurate.
Before level 32, slotting for endurance reduction isn't an issue. After level 32, the Demons and the Demon Prince need endurance cost reduction lovin'. Specifically, the demons and the demon prince have abilities that ARE NOT affected by the endurance cost reduction portion of Pet damage IO sets. Ember demon needs endurance reduction for the single and AoE heals, and for the resistance aura cost. Hellfire Gargoyle has a resistance debuff aura endurance cost. Demon Prince has a lot of non-damage abilities like shiver and his PbAoE slow aura. Fortunately most of his attacks have slow components, so I recommend grabbing four Pacing of the Turtles that include accuracy, damage, slow, and endurance cost along with two other pet damage IOs to increase accuracy and damage. Four PotT also includes a 9% accuracy to your other ablities like heat exhaustion 
When you are high enough in level, you can slot Hammi-Os. These work great in Demons. Last I checked, centrioles are fairly inexpensive. These affect damage, range, AND damage resistance. The unintended effect to damage resistance has been talked about but the devs have known of the situation for years and have not tried to 'fix' that bonus. Two centrioles and two golgi's will maximize damage, damage reistance, and heal. The golgi's also address the endurance needs of the ember demon in his heals.
I don't mind discussing Demon Summoning. The forums get this question weekly. It should be stickied some time.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
oh yeah: Part DUH
For Demons, I find it easy to fit a Pet Dmg Resist or Defense IO bonus enhancement in the Demonlings if you like. The accuracy and damage can be maximized with five slots. Hell on Earth needs at least three of the four of the bonus enhancement IOs. I also like to max out the recharge in Hell on Earth so that the recharge time is as low as possible.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
So, I've whipped up a build (leveling at least), for my Demon/Thermal, but I'm not sure what to slot in the Demon powers themselves (Summon Demonling, Demons, Demon Prince).
A friend of mine puts some of the Pet Uniques (the Def/Resist bonus ones) into the various pets, while other people (at least here) claim it's better to put ALL the pet uniques into Hell On Earth.
Are there any suggestions as to combinations of slots/sets?