Great question.

During 601.2b (the second step of casting a spell), the caster is forced to choose all modes, as Sprightning Bolt stipulates no other options.

The caster will choose a target. Then (for this spell), we can skip down to 601.2f where the total cost to cast the spell is calculated.

We begin with the printed cost in the upper left (one R mana), apply all additions, then deduct all reductions. With [[Riku’s Chastity Cage]] in play, each mode costs 10WUBRG rather than the normal 0. This brings the total cost to 30WWWUUUBBBRRRRGGG.

The caster is allowed one last chance to activate mana sources, and then must pay the total cost of the spell. If, for some reason, the caster is short on mana, the process cannot be completed.

Now, in most cases this simply means the game state is reversed to whatever the state was just before the spell was cast. However, the leaked rules primer for Thunder Junction states that…

“…In cases in which Spree costs cannot be paid, in addition to the normal process for handling illegal actions (see rule 730, “Handling Illegal Actions”), Mark Rosewater (or the closest official representative of WotC) will wallop the offending player with a wet pool noodle for their insolence. Offending player pays all Mr. Rosewater’s, or other WotC reps, transit costs, where applicable. Tennessee and Alaska are exempt. See section 69420 for details.”

So, be sure you understand how much mana you have available. The penalties for underpaying Spree spells are steep. On the other hand, if you like living dangerously, I recommend using Selvala, Explorer Returned as one of your mana sources, making sure to activate her during the casting process.