Always check the price on parts from the dealer first. Every now and then you'll find them to be the cheapest place (especially for items specific to that brand of car).
One item in particularI'm running off memory and estimating the pennies, but I remember this one well enough to be close).
Cam Cover gaskets, set of 1R and 1L, for 3.8, 4.2, 5.0, and 5.6L SOHC Mercedes V8 1981-1991
Discount Import parts: $73.44
Advance: RH $22.88 LH $24.88
AutoZone: can't get them
Pep Boys: RH $35.00 LH $35.00
(all of the above had to order them except DIP who had them in stock).
MB dealer: RH $9.11 LH $7.93
I did ask DIP if his were rubber or composite and he said neither, they are cork.
MB's were a new part number superceding the earlier composite ones. The new ones were nitrile rubber with a greenstripe sealer already on them.
You just never can tell...