Wolf,
my estimate cost of the material are approximately 50 euros. put a double eagle M4 in it,the cost are c.a. 100 euros. whats left is the other Barrett, and in the other Barrett can be used for a gearbox of any choice when we got more information from the others (all of you).
And the better version must still be purchased loose rails, hand grip etc. etc. so that will be more expensive (c.a. 200-250 euros)
So maybe after the making off the cheap version i can sell it and buy a good gearbox and other materials