I got a smoking deal on a Specwar 556 on Black Friday. 400 with the Flash hider. Sold the flash hider that day for 50.00. Net 350.00 for the can (plus tax on 400). I got it at Blackwing. They stock the ones they sell and they have always matched or beaten silencershop.com's price. AND, you can shoot it while it's in "jail".
In reality, I think Gunenvy already has low prices. If you don't care to shoot it while you wait I would give them my business. But, shooting it is a nice perk for me.
And finally, Most places charge between 50-200 (Gunenvy-Blackwing) to do an NFA transfer... SO, if you order a can from Silencershop.com and have it shipped to Blackwing (I believe) it is 200.00 for the transfer (and then 200 for the tax stamp obviously). 200.00 is negligible on a machinegun... but not on a suppressor.
Gunenvy charges 50... and I'm sure blackwing chargers 200 because you "can shoot it while it sits in jail".
I don't find the sense in having a suppressor transferred as most dealers should be within 50 bucks of the lowest price you can find online.