The prices for the D and maybe the E (if the price is $7 or less for the E) seem semi-reasonable in that they are around what it costs for a Aerotech AP composite RMS24 reload in the same impulse class.
The proposed $12 price for a 55 to 60n-sec 45-50% F is considerably higher than the street price for virtually all 80n-sec full 100% F RMS29 reloads. That's WAYYYY too high $$$$ for me unless they can get the impulse up to full F level (which can't be done with BP under the 62.5gr limit).
SU BP engines should cost less (or at most the same as BP and cardboard is FAR cheaper than AP propellant) than an equivalent RMS reload in order for me to be interested in them for anything beyond buying very few to satisfy curiosity unless I'm flying a "fire-n-forget", "send it into orbit " junk heap of a rocket.

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When in doubt, WHACK the GAS and DITCH the brake !!!

Yes, there is such a thing as NORMAL, if you have to ask what is "NORMAL" , you probably aren't !
Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY.
ALL systems are GO for MAYHEM, CHAOS, and HAVOC !