So, I understand 3E perfects take something like twenty motes now. Is that right, and would you say it's still worth perfecting an Ambush?
That's not remotely true.
The three old perfects now work like this:
Seven Shadow Evasion costs 4m, 1wp and perfectly dodges any one attack, but can only be used once per scene unless you dodge three decisive attacks from dangerous opponents by using Reed in the Wind.
Heavenly Guardian Defense costs 4m, but doesn't negate the attack by default. Rather, it removes damage on a decisive attack at a cost of 1i per negated success.
Adamant Skin Technique costs 8m, and allows you to apply your full soak to decisive attacks (normally, no soak applies there at all), which makes it very hard to injure you (Stamina 5 and Heavy Artifact Armor would give 19 soak).
Note that all three of those have a weakness.
Seven Shadow Evasion can only be used once, that reset is rather hard, especially against a powerful single opponent.
Heavenly Guardian Defense depletes your Initiative, so you might not have enough to completely negate an attack.
Adamant Skin Technique is expensive, but a really high-damage decisive attack can also power through it.
So they're not really "perfect" anymore, except for SSE, and that one can only be used once.
Note that there also charms that interact and enhance those "perfects", which also don't cost any motes or willpower.
Refinement of Flowing Shadows gives you one point of Initiative each turn after using SSE, until you get hit by an attack or enter concealment/disengage to extreme range.
Protection of Celestial Bliss notes the 1's and 2's whenever you use HGD, and gives you 1 point per that you can spend instead of Initiative for HGD. You can also double the cost of one HGD to double those points for it.
Aegis of Invincible Might increases Hardness to 20 (!) and soak by (Stamina + Resistance, so likely 10) after AST is used. You have to pay 8+ motes on offensive charms or unleash a decisive attack each round to maintain it, but it turns you into a pretty hard to stop juggernaut.
All of these are Essence 5 charms, so you better have the relevant ability as a Supernal.