My $0.02:
The M11/NINE, the M11a1, M10/9, and M10/45 are already pistol caliber subguns. I would ignore any probable semi-sales...if they occur it would be a very small number.
Because they already shoot pistol calibers the attraction for a product you would produce for the subguns would be for rifle caliber - either magazine or belt feeding. THe most inexpensive, common, and popular rifle caliber is 5.56mm.
I honestly question whether R&D costs ($125K+?) would be recovered within a year for a 9mm belt fed version upper for 9mm (or .380 or .45) sub-machine guns. The M16 being a rifle caliber MG has a much greater potential sales target for a 9mm belt fed upper.
That said, if you could design, debug, and get BATFE Tech Branch approval on a rifle caliber upper for the M11/NINE platform, which is the most populous of these types on the registry, and offer it for $2500 or less I believe you could recover R&D within the year and accrue profits thereafter.
The cost to customer is cost per unit + R&D/x units + profit per unit until R&D is fully recovered. Dropping price (rare but not unheard of) after R&D is recovered to stimulate sales is a possibility. I'm not telling you anything you don't already know - I'm just laying it out for all to understand.
Your question at the start of the thread was in regards to an M11/NINE version of the FM-9 belt fed upper. To get some sort of answer that is even in the neighborhood of helpful would probably best be done with a poll. Said poll should question -
"I would buy an FM-11/9 for no more than $3000"
"I would buy an FM-11/9 for no more than $2750"
"I would buy an FM-11/9 for no more than $2500"
"I would buy an FM-11/9 for no more than $2250"
"I would buy an FM-11/9 for no more than $2000"
"I would buy an FM-11/9 for no more than $1750"
The poll should provide some insight into what this small group on UZI Talk - which is a small segment of the SWD/RPB/Ingram subgun owner pool - would be willing to pay for an FM-11/9.
As an earlier poster noted, the real proof of whether someone is talking crap or is serious about purchase is to get them to commit some deposit. Either a token non-refundable or a serious fully refundable deposit.
Regarding a rifle caliber possibility, I would start another thread for that subject and keep this one for the 9mm belt fed upper only.