You're locked out of weaponskills for 5.0 seconds while performing Standard Step. Therefore, you need to account for the potency of the weaponskills you would have been using instead.
Cascade: 200
Fountain: 250
Reverse Cascade: 425
Fountainfall: 475
Reverse Cascade and Fountainfall have a 50% chance to proc Fan Dance 1 (150p), which then has a 50% chance to proc Fan Dance 3 (200p). So the expected potency of each of these weaponskills is 150*0.5+200*0.25 higher than listed.
Since Cascade and Fountain have themselves a 50% chance to proc each of their flourishes, there are four possible outcomes:
Cascade(200), Fountain(250): 450/2 → 225 per GCD
Cascade(200), Reverse Cascade(425), Fountain(250): 875/3 → 291.7 per GCD
Cascade(200), Fountain(250), Fountainfall(475): 925/3 → 308.3 per GCD
Cascade(200), Reverse Cascade(425), Fountain(250), Fountainfall(475): 1350/4 → 337.5 per GCD
The average of these four outcomes is 290.625 per GCD. At a GCD of 2.41s, this is 120.6 per second.
Standard Step takes 5.0s to execute, so it locks you out of 603 potency worth of weaponskills you would have been using. Therefore, it's really only worth 397 potency per use.
(This is still a big deal; just not 1000p big.)
(Edit: I prolly should have just added all those individual potencies and divided by 12, to account for each possibility having a different length. In that case, average potency per GCD would be 300, Standard Step would lock you out of 622, and it would only be worth 378 per use.)