Just from my observations of PGC to credit conversion, as well as any item mall purchases that are buyable via the black market in game, the prices don't always make sense - or are entirely dependent on the availability of said item. Anyone coming from Void will probably have a hard time getting their head around the fact that you can't put prices down to a 10:100 (as an example) and use this as a copy-paste for any and all transactions. Won't work like that here as the prices do not adhere to that format.
[Info from above listed below]
$5 - 100 to 150m
$10 - 200 to 300m
$20 - 500 to 600m
$30 - 750 to 900m
$50 - TBD
$100 - TBD
"can change anytime"
The prices listed on the discord do generally appear to be outdated* [outdated when taking into account influx of players - yet more up to date than my observations as I have been informed], I did monitor prices for quite a while and do my own. The prices I observed ranged from lowest to highest in terms of what I saw being sold or bought, they were not figures I manufactured.
(Couldn't help but notice you left out $75 from your pricings...?)
PGC Prices as of 9th of July 2019
$5 - 1500bits = 90-170m
$10 - 3600bits = 250-400m
$20 - 7800bits = 400-800m
$30 - 12,000 = 580m-1.2b
$50 - 21,000 = 960m-1.5b
$75 - 32,700 = 1.8-2.5b
$100 - 50,000 = 2b-3.1b
Your prices are listed as being an update as of yesterday but I still can't help but feel its off base for the aforementioned reason that you're trying to apply void rate logic to here, where its a bit more freeform.
Now onto the guide portion since your post wasn't entirely about price analysis. PGC to credits is guaranteed, you're getting a rate for a code. Whether this is fair or not is entirely up to the two parties doing the transaction, whatever they negotiate is on them as it will not always conform to whatever prices are set by others. If people run the gacha themselves I believe the only people that benefit are costume hunters or the general player on C:C. Let's indulge in that hypthetical for a moment: EVERYONE stops selling PGC and everyone gachas, market becomes flooded with costume pieces, which can go one of two ways: a) market becomes saturated and prices go down b) sellers take a hard stance and prices rise.
There is no 3rd direction for this because if a massive influx of costume pieces happens it won't stay the same - people will want their money and the method you have laid out isn't a guaranteed way of making money. Gacha is literally just a gamble, whereas PGC code sales are not, you get a flat fee for a sale.
And that golden ratio? Sigh...
(As an addendum - what your post really feels like is a way to boost PGC prices so you get more bang for your buck... Also, you did not factor in that someone could advertise selling $50 and then just give x10 $5 codes... It would be perhaps viewed as petty and spiteful or as funny by others but this would counteract those bonuses if that was a legitimate concern on the part of the seller)