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Rebuilding your front seats is not the best project to start working on cars, but it isn't that hard. With some tools, good instructions and some coaching, you can do it.
If the seat covering material is in good shape, that is good new$. It would mean that at least the seat cushion needs to be replaced and most likely the seat bottom spring as well. We like to support this site's host by buying parts from FastLane. Good deals, good people and good service.
The seat backs do wear, but often doing the lower cushion can get you back to a good feeling and working seat.
There are multiple postings on this site about such repairs. It is best to use the search button in the upper right with several words. To get this search for you I used REBUILD SEAT .
The cushion material varies but often will run a little less than $100. The seat spring is a little less than double that, depending of course on where you buy it. The seat spring is a unit consisting of the springs and a metal frame. Everything must be removed from it, including the seat motors, cables, seat back, covering etc.
$300 for the cables sounds like a lot of money, but there may also be a motor involved. I would consider the two repairs (cables and cushion and/or springs) as almost separate repairs although since the seat is out, it is much easier to do it all at one time.
I think that a lot of us find working on our cars interesting, often fun, provides a great sense of satisfaction and can often save one a lot of money. If you don't think you can do it .... ask more questions until you really understand, then decide.
Keep us posted,