If you're looking to make TWF cost less an idea might be to make it that one-handed weapons only cost half as much to enchant (E.G 1000 gp for a +1 weapon).

This means that someone buying an enchanted...
  • One-handed weapon and shield
  • Two One-handed weapons
  • A two-handed weapon

...all cost the same to enchant. Plus, in my opinion, it makes enchanting weapons a little more realistic (seems a little off that applying magical properties to a small dagger costs as much as applying the same properties to a golaith greatsword.)