While champions do sometimes get put on sale, the blue essence prices required to purchase them are not lowered, only the Riot Point costs are.
Do note, however, that in addition to a, perhaps quite lengthy, wait for the champion you're looking for, you will also have to pay real money (in Riot Points) to even take advantage of it.
The mechanic Riot has introduced for reducing champion blue essence costs over time is as follows:
- Champions are released costing 7800 blue essence. After one week, this is reduced to 6300.
- With each new champion released, the oldest 6300 blue essence champion is reduced in cost to 4800.
- Every third champion release will also mark the reduction of an older champion by one price tier (4800 -> 3150, or 3150 -> 1350).
Riot releases the schedule of which champions are up for price reduction in advance, so there should be few surprises on that front (though now that their time between champ releases has doubled, it'll be much longer between price reductions).