Just re-upholstered my Chyrsler last fall, ordering through JC Whitney. Cost was about $250. What I got was a set of tweed/vinyl custom made covers that fit my model. Opel would be less because it is front seats only. One half, but less. Plus, the headrest is an extension of the seat bcak, so you don't have to pay extra.
These covers can be installed over the existing vinyl, or, you can strip the old ones off, and use spray adhesive to install new ones. But chances are you will not get the full bolster effect with these covers.
Years ago, I reupholstered my wife's GT, using the old covers for patterns. I sank about 50 hours into it, and while it turned out good, and kept costs to under $60, it was difficult and time consuming.
Its a balance of how good you want it to look, and how much you want to spend.
Friend of mine lives by the philosophy, "if its going to show, pay to have it done".