Typically, it has taken 1,500 credits worth of spend on whatever game or pack offering is available to fill the SEC progress bar. Bonus packs, wheel spins, scratchers, progressive discounts, whatever. 1,500 credits.

A couple months back, I dropped 1,500 credits on whatever new game came up and not only did I fill the bar, I filled it again for a gift card, and made progress toward another one.

Since then, I've been checking each new event to make sure what I was seeing was a pattern, not just a miscalculation or short-term offer. With the new Pleasure Chest I can confirm that the new target to fill a progress bar is 900 credits.

Note that this is outlay only. With my 900 credits spend, I received a back catalog SEC card, a gift card, two free cards, and 620 credits own back; plus the new SEC card for event progress.

Correct,
the credits Spent to fill the Bar has been reduced for the Past couple of events.

900 to reach the SEC or 1st Bar fill up
600 to fill the bar the 2nd Time
500 for the Next and Subsequent fillings

The July Event
The August Event
and now the October Event

900 credits Spent to fill the Bar.

And Just as you Note:
While Playing the Event, you will also Win Back Various Prizes, Including an Amount of Credits.
These Winning Credits Offset your Total Credits Spent.
So in your case example
You Spent a total of 900 credits, but along the way you Won Back 620 credits
so your Net total was 280 credits spent to fill the Bar.