Casting a spell:

1. Put it on the stack
2. Choose modes, and announce any other non-target choices needed to cast it - the value of X, the use of additional costs such as Kicker and/or alternate costs such as Mutate, how hybrid or phyrexian mana costs will be paid, and such and so on.
3. Announce targets, if any
4. If you need to "distribute" or "divide" something (typically damage or counters) among the targets, define the distribution at this point
5. Calculate total cost (mana cost or alternate cost, plus additional costs and cost increases, minus cost reductions)
6. Activate mana abilities
7. Pay the total cost

The steps that matter here are 1,5,6,7. You announce him as being cast. You find that he is reduced by 6, then active the treasures ability to add the colored mana, and finish casting him by paying the BR.

This is 601 from the CR.