It is a rough approximation.
If SPTech want to sell at 1000$ we will buy 175 units.
If SPTech want to sell at 1500$ we will buy 175 units.
If SPTech want to sell at 2000$ we will buy 175 units.
But...if SPTech want to sell at 4500$ we will buy 0 units aprox, because no ROI.
Law of supply and demand.
I announced "I hope to set the final price on July 15 based on the difficulty of the moment.."
but many people want to give me his/her opinion on the price by PM.

I know the price we want to pay (the minimum).
SPTEch know the price you want to sell (the maximum).
If negotiation is not over I can not roll the dice and give a price. Please keep patience, I said 15th July.
This makes absolutely no sense.
So basically you are going to exclude people randomly according to what you consider a "good deal"?
Will you be trashing the entire "group buy" when the final price ends up more than $3800/sp30?
Your estimations are so far from reality why should you decide how many units the group buys? (production costs of an sp30 are probably more than $2000)
Anyways if you do manage to negotiate the price down to $0 per sp30 count me in for 200.