What is the average cost for replacement? Thanks.

One more thing... my car feels less stable at higher speeds. When flooring it, and car shifts from 2-3 at about 60 mph, car feels squirrelly. Also, that popping sound I get when accelerating.
Are these symptoms of worn bushings?

(Thanks again Steve for the explanation.)

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)