It's fairly cheap. The mount is $8.50 (each) + S&H from fcpgroton.com. The garage might charge you an hour of labor. You should replace both (I would even go further by replacing the trans mount). You need to do it right away, unless the mount is still somewhat attached and not ripped off completely. As far as wors case scenario; the other mount could tear out and the engine would be free to move back and forth, ripping electrical cables and what-nots you can imagine! In other words, don't put it off!!

I did the install myself, which took me less than an hour each side. If you can turn a wrench and use a jack, you can replace them. You will need two jacks, one for raising the car, the other to raise/lower the engine so that you can align the mount with the block cross-member. I do recommend using a piece of 2x4 as you will be jacking under the oil pan to move it up/down. The piece of wood will spread the pressure and not concentrate it on one spot, which can put a hole in your oil pan.

Cheers,
SM
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90 BMW 325is, 98 BMW 528i, 90 Volvo 240DL