I found the error. If average is 1000% mod increase, you have to multiply that by 5 for each weapon class, so really a 5000% stat increase.
That makes it a 6.8B/7B x 5 = 30B/35B increase.
I just bought 1 of each and my total stats went up by around 480M/276M... for 6.6k gold . Ya, that's a rip off. I'll see what the money weapons do