1e
Here's an old idea, from 1e AD&D.
Non-magical +1 Weapons: Made of fine steel, or other ordinary materials. 2x the normal price (in 3.5e, that would pro'ly have to be the MW price).
Non-magical +2 Weapons: Made of even finer steel, perhaps alloyed with exotic materials, or other, finer, ordinary materials. 5x the normal (MW) price.
Non-magical +3 Weapons: Made of Meteoric Iron, or Mithril-Alloyed Steel. 10x the normal (MW) price.
Non-magical +4 Weapons: Made of pure Mithril, or Adamantine-Alloyed Steel. 25x the normal (MW) price.
Non-magical +5 Weapons: Made of pure Adamantine. 50x the normal (MW) price.
All are MW, and suitable for enchantment. All provide the plusses To-Hit, as well as to damage. Craft DC:25+ plusses. Note that this defies the usual MW and Materials costs, in the DMG. These would be relatively easy to make, and as available as the materials required to make them. Thus, +1/+2 weapons should be common.