They have been known to give a 5% discount if you spend over a grand, but there are cheaper places. Joe Blow Racing has consistently had the best deal on their coil-overs... couple hundred cheaper. However, many vendors will match that price if you call them and ask.
MM has stated to me on the phone that they do not want to undercut their vendors. So basically your best bet price-wise is through a vendor. Livernois Motorsports has told me twice they will match other vendor prices. Of course, peace of mind goes a long way with big purchases and you get lots of that for free from MM directly.
Another vote for stangsuspension.com. Josh is the owner and he is very good with his price. Give him a call, he might give you a better price on the phone. Tell him you saw his add on corral net.com.
I ordered parts from Art and a few days later got a quote from stangsuspension.com on the same parts (springs and CC plates) and stangsuspension was cheaper