Lethargy Trap — ruling 1

You may ignore a Trap's alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.

Lethargy Trap — ruling 2

Casting a Trap by paying its alternative cost doesn't change its mana cost or mana value. The only difference is the cost you actually pay.

Lethargy Trap — ruling 3

Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.

Lethargy Trap : règles, fonctionnement et interactions (MTG)

Lethargy Trap is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

If three or more creatures are attacking, you may pay rather than pay this spell's mana cost. Attacking creatures get -3/-0 until end of turn.

How it works : Lethargy Trap

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
You may ignore a Trap's alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.
No other card is named in this ruling.
#2October 1, 2009· Wizards of the Coast
Casting a Trap by paying its alternative cost doesn't change its mana cost or mana value. The only difference is the cost you actually pay.
No other card is named in this ruling.
#3October 1, 2009· Wizards of the Coast
Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.
No other card is named in this ruling.