dnd 5e – ‘Advantage’ or ‘Disadvantage’ on damage rolls

I am looking at creating a few new categories of weapons just to add a bit more reason to spend money on mundane equipment in the early game. Poor, and Masterwork.

My initial idea is that poor weapons will use the disadvantage mechanic on damage rolls, that is rolling twice and taking the lower, and masterwork weapons will use the advantage mechanic.

They won’t be magic, and I will probably make poor and masterwork versions of magical weapons as well, but I don’t want a masterwork weapon to overshadow a magical +1 weapon.

When rolling a D20 the accepted average seems to be that advantage equates to a +4/+5 bonus, so what would that average bonus be on damage rolls? Given the different damage dice it is probably best to stick with a 1d8 weapon in this case, but I would assume the advantage is less for small die, and more with larger die.

A good answer might include an anydice table or some kind of maths, but I find all that really hard to get my head around, so an answer along the lines of ‘advantage when rolling a d8 equates to roughly x’ is the best for me, with explanations in words rather than maths if you need to add anything else.

A really good answer will include anything else that I might need to consider but haven’t added to the question.

Note: I have seen this question, which is similar, but if the answers there happen to cover my question I don’t understand them.