Start by replacing the weatherstripping - it's way cheaper and often that's all you need. Rubber or foam seals wear out over time, especially on older PVC doors. You can find them at hardware stores or online, and they come in adhesive rolls that are ready to stick on. Clean the frame really well before applying the new seals, and remove any leftover bits from the old one. If you install them properly, that should block the drafts.
If it's still not enough after changing the seals, then you'll want to look at the door itself. It might be slightly warped or misaligned, which creates gaps. A carpenter can adjust the hinges or check that everything is square. As for prices, honestly it varies a lot depending on your area and which carpenter you go to, but replacing just the weatherstripping is really affordable (just a few euros for the materials), while replacing the whole door costs way more. Try the seals first - you've got nothing to lose.