Adding mana cost of a spell

[Lorn Asbord Schutta]

Llanowar Wizards
Creature - Elf Wizard
: Add mana equal to target green spell you control's mana cost.
1/1

If I understand 601. properly, this above ability would not differ in practice much from "you may cast a green spell without paying its mana cost".
I seek a way to add mana equal to spell's mana cost, that would at the same time prevent it from being self-fueling. I thought about something along "Choose a spell you control. As that spell resolves, add mana equal to its mana cost.", but this would mean that fizzling or countering would interfere with it.

[void_nothing]

Well, no, this does not work in such a degenerate way because it doesn't have mana ability timing. Simply put, it's not a mana ability. CR 605.1a says in part that an activated ability is only a mana ability if it does not require a target (among some other requirements). If you really want to spell it out, you can say (Activate only any time you could cast an instant.)

[Lorn Asbord Schutta]

That is good to hear.
But what about:

Lawnmower Wizards
Creature - Wizard Gardener
, Exile a spell you control: Add mana equal to exiled spell's mana cost.
1/1

This one would lead to things like putting Enter the Infinite on stack on turn 2, then using this ability to generata twelve mana out of nowhere, right?

[WizardMN]

Technically, yes as there is nothing in the rules that says it is illegal to start trying to cast a spell you might not be able to pay for later. However, if you can't pay for it, everything would be rewound if you can't complete certain parts of the process. Importantly, you would announce the spell, use this to generate the mana for it and then you would need to use that mana to pay for that spell. Which won't work out well since that spell is exiled which means you would pay for it but it would never resolve.

If you can't pay for it for some reason (perhaps your ability only generates colorless mana or something) then the announced spell is rewound and mana abilities are undone as best as possible. And since this is moving an object from a public zone to another public zone it is trivial to rewind which means you still don't get mana from it.

So, no this won't let you cheat a bunch of mana.

To an earlier point, if you want to ensure that you don't need to worry about mana ability weirdness and don't want to deal with a counterspell stopping you from getting the mana, you could word it as "the next time you cast a spell, add mana" (obviously wording it correctly). This makes it not a mana ability and means it still works even if the spell is countered. But also means it can't fuel itself since the trigger can't go onto the stack until it is cast which means after it is paid for.

[Lorn Asbord Schutta]

Ok, so if the effect would be:

Apprentice Managiver
Creature - Wizard Druid
: The next time you cast a spell, add mana equal to its mana cost.
1/1

And then say, there is a Shock on stack and you want to cast Negate. You now get priority, first you activate Apprentice Managiver ability, let it resolve, and only then you cast your Negate to get back your mana, right?

And back to ", Exile a spell you control: Add mana equal to exiled spell's mana cost.", it would not let itself to degeneracy due to reasons mentioned up, but it would work as intended with spells that were already paid for, if I understand correctly. Is there a way outside of some funky reminder text to steer players away from trying to use this to cheat mana and then failing miserably?

[WizardMN]

You would need to word it as a trigger (the wording you used above isn't a valid template) but otherwise your summary appears to be correct. You would activate Managiver, cast (and pay for) Negate, and then get the mana back for the Negate when the trigger resolves. Effectively, it refunds the mana used for Negate but still requires you to have the mana up front first.

I guess it depends on what is "intended". The previous wording is going to ensure that the spell you exile will not resolve. Which seems to make it mostly useless except if very edge cases as there isn't much of a reason to pay for a spell and then exile it to get the mana back. You would be better off just skipping the exile spell part and paying for the next spell directly.

Due to a few different issues with that wording, including what players will and won't think is possible, I would recommend just avoiding it entirely. I don't think there is an elegant enough reminder text to indicate what is and isn't possible with that and, more often than not, players are going to think the exiled spell is still going to resolve. Mainly because of the point I made above: the ability has no real use if the exiled spell doesn't actually resolve.