$250 is ridiculous. Are you sure there's not some communication error with the parts place? Google for "86 300SDL engine mount" and you'll find some for $25 or so each. It's best to replace both.

It's important to do it quickly, because as the rubber tears apart, it can dirty the hex bolts and make it harder to get them off. Little grits of rubber stuck inside the hex bolt can make it harder to get a solid attachment and make it easier to strip the bolt. Happened to me and I was forced to drill out the bolt entirely to get my new motor mount on. It's important to clean out the (female) bolt using a drill bit, etc. The motor mounts use female/inverted hex bolts (ie allen bolts) that require special sockets.