Least expensive? Go to Walmart and buy some Coleman sleeping bag mats (the eggcrate ones). Two layers (eggcrate together) glued and top part folded over to match the factory size, sew down to spring with cotton string (carefully or the string cuts right through). Go to the fabric store closest to you and check out the cut-rate section, find some heavy vinyl close to the correct color and sew up a set of covers using the old ones as patterns with 1" foam instead of cotton padding. You may want to use the thinner closed cell foam polypropylene mats as a base, too. Did one front and the rear seat in my brother's 300D this way.

Best? Buy new pads and covers from World Uphholstery or Adsit, etc. Be prepared to lay out some cash for this ($1000 per seat up front, $2000 for the rear in leather, cheaper in Tex).

Middle of the road: buy used pads in good condition (or used seat in good condition) and replace your rotten ones, make new covers from Italian Upholstery leather (this is what I did -- pads were fine). Use garment grade leather if you can find it, the upholstery leather is a bit thick and harder to manage.

See pics from the vinyl seats I did last year -- leather in the works.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!