Are you allowed to choose to not pay costs for a spell that you started to cast?

Question

Say I start the process for casting a spell, and activate Selvala, Explorer Returned as part of the process of casting the spell. I have plenty of lands available to cast the spell whether Selvala gave me any additional mana or not. But due to something that was revealed when I activated Selvala, I now know that casting the original spell is a bad idea. Can I simply choose not to produce enough mana to cast the spell, thus having it reversed and returned to my hand?

I'm aware that I could have just activated Selvala before starting to cast the spell; and cannot think of any legitimate reason why I would begin to cast the spell first if there is a chance I would not want to follow through with it.

Answer 1

No, you cannot choose to stop casting a spell once you have announced that you want to cast it. The only way to stop casting a spell after that point is if you are unable to make the choices required by the spell, or if you make a game play error by misjudging your ability to pay its total cost. Either way, Selvala's ability has no relevance to this as it cannot be reversed.

The Comprehensive Rules do not provide a voluntary exit point for the process of casting a spell. Once you did start casting a spell, you follow all instructions as outlined. The only premature exit point in the CR is when a spell becomes illegal to cast, at which point the game is rolled back to the state just before you started casting. This has been discussed in this question

601.2e The game checks to see if the proposed spell can legally be cast. If the proposed spell is illegal, the game returns to the moment before the casting of that spell was proposed (see rule 726, “Handling Illegal Actions”).

This happens after making choices, but before determining and paying costs.

If you made all required choices legally but then determine that you can't pay the total cost (typically because you don't have enough mana available), you have committed a game play error as per the Infraction Procedure Guide, with appropriate steps taken by the judge to remedy the situation and/or give a penalty as deemed appropriate.

That being said, Selvala's ability has no bearing on any of the above questions. Selvala's ability, once resolved, cannot be reversed. It does not matter whether or not you activated it while casting a spell, or whether or not casting that spell got cancelled.

726. Handling Illegal Actions

726.1. If a player takes an illegal action or starts to take an action but can’t legally complete it, the entire action is reversed and any payments already made are canceled. No abilities trigger and no effects apply as a result of an undone action. If the action was casting a spell, the spell returns to the zone it came from. Each player may also reverse any legal mana abilities that player activated while making the illegal play, unless mana from those abilities or from any triggered mana abilities they caused to trigger was spent on another mana ability that wasn’t reversed. Players may not reverse actions that moved cards to a library, moved cards from a library to any zone other than the stack, caused a library to be shuffled, or caused cards from a library to be revealed.

Answer 2

You are obliged to pay the cost if able, but you are not obliged to activate any abilities to do so:

118.3c Activating mana abilities is not mandatory, even if paying a cost is.

Example: A player controls Lodestone Golem, which says “Nonartifact spells cost {1} more to cast.” Another player removes the last time counter from a suspended sorcery card. That player must cast that spell if able, but doing so costs {1}. The player is forced to pay that cost if enough mana is in their mana pool, but the player isn’t forced to activate a mana ability to produce that mana.

Paying the cost of a spell isn't optional (note the lack of "may"):

601.2h The player pays the total cost. First, they pay all costs that don’t involve random elements or moving objects from the library to a public zone, in any order. Then they pay all remaining costs in any order. Partial payments are not allowed. Unpayable costs can’t be paid.

If Selvala's ability generated enough mana to pay for the spell, you must use it. If not, you aren't required to continue playing the spell.

I agree that I can't think of any particularly good reason to do this, compared to just using Selvala's ability (and not revealing a card from your hand that you might want to keep hidden until later), and I wouldn't say it's a particularly polite or sportsmanlike thing to do, but it follows the letter of the rules.