$10 halloween costume prop? Hollow plastic, super light.
Training weapon for self defence against knife attacks? Solid plastic, several times the thickness of a real knife, probably bright blue.
Olympic fencing weapons, which simulate duelling? Pretty light, you only need a light touch to score a point and bending is desirable.
Live Action Role Play where they want to whack each other, but not hard? Anime cosplay? They mostly go for foam-covered plastic which is kinda light, but they're often thick and also they often simulate unrealistically large weapons, so they can end up kinda heavy.
Ornamental/replica things that are just for show? Could go either way. You could make an extra-light sword out of aluminium, or an extra-heavy ornate dagger out of pewter. Less steel means a lower material and shipping cost - but a bit more heft that feels realistic to buyers might get you better product reviews.
Apparently some HEMA/renaissance fair folk wear full suits of metal armour, metal longswords that are blunt-but-authentically-weighted, then try to whack each other pretty hard? Looks like concussion city to me.