Rules are always to make things happen the same way twice.  

For this kind of thing I think you need to wing it.  

You said you don't want to do that and I can see that but any system will be bound and confined by its self.  Real freedom of imagination needs to go beyond the rules.  

That said,  you could make your self to table to assist.  

I would have them divided by elment, probably 5 or 7 easyern type elements instead of the 4 greek ones.  So you can have wood and metal and void as well as fire earth air and water.  

Power source.  Deity, lay line, pact, personal will.  Blood scarifice, elemental conjunction, celestial conjunction, ritual, festival, orgy.

Then power level.  

Then probably throw in a bonus randamizer.

The key is not to roll.  Or only roll for inspiration.  Toss out any effects that don't fit, add anything that you think does fit try to add something that isn't on any table.  

As for cost I think to be fair they need to have a clue about cost but not know.  Also its not fair to charge a perminate price for a tempoary effect.  Bringing down a stone bridge to stop persute could cost level of exhaustion, or make you unable to cast spells for days or weeks, but its not fair to charge perminate spell slots or hit dice.  

But you you cast a spell to bind a deamon to your service then sure that could cost a clerics turn undead ablity for example.