Can you use treasures generated by a spell to help pay for that same spell?

When you cast a spell, you must pay its mana cost as the final step of the casting process (CR 601.2h). This means you need to have your mana already available — you cannot use effects that the spell itself generates during or after resolution to pay for it.

Treasure tokens are permanents that enter the battlefield only when a spell or ability resolves (CR 700.4). Since a spell's resolution happens after you've already paid for it, any Treasures that spell would create don't exist yet when payment is due. There's no way to use them to fund their own creator.

It is worth noting that if something else (like a triggered or activated ability) creates Treasures before you begin casting a spell, you can absolutely use those existing Treasures to pay for a new spell. The restriction is specifically that the Treasures must already exist when you reach the payment step of casting.

Example: You cast Spell Swindle (which creates Treasures when it resolves) targeting an opponent's spell. You cannot tap future Treasures Spell Swindle will create to help pay Spell Swindle's own 3UU cost — those tokens don't exist until after it resolves. You must already have 3UU available from lands or other mana sources.

The relevant rules are CR 601.2 (the casting process steps), CR 601.2h (paying costs), and CR 700.4 (tokens entering the battlefield upon resolution).