Sorry if my reply sounded a bit rude but, the link I suggested (from the search function) has literally tens of references of prices of replacing the flex disc along with suggestions and recommendations for DIY on this subject.
I did not assume anything, I simply refered you to a link where you could see (and read) more on the subject.
JackD

Just a little tip...
 

I finished this job yesterday and I may be stating the obvious, but for those who attempt this in the future, realize that the bolts are two different lengths. I found out the hard way when the shaft wouldn't turn without hitting something. (longer bolt was in shorter bolt hole)

I was able to pry the shaft back, to give me room to change out the disc. Easy DIY job as Mike stated above.

Good luck :D

How does one know it's time to change
 

the flex disc front or rear??? is this a visual check or something starts acting wrong before you decide to make the change??

the flex disc front or rear??? is this a visual check or something starts acting wrong before you decide to make the change??

You can Visual Inspect them at every oil change

Also listen for the CLUNK when Put into reverse or drive!

The clunk woud be obvious, but
 

what exactly would you be looking for when doing a visual inspection?

You want to look for any play in the driveshaft as you twist it back and forth just aft of the disc. (rear wheels on jacks -- front on ramps)

Also look at the flex disc bolts and the metal sleeves they insert thru and look for some looseness there.