I've got the chance of getting some custom made bar end weights, what's a decent weight to get?

Same as OE?, the weight is usually tuned to damp out resonance / vibrations...
unless they are purely cosmetic and to stop the handle bars acting as an apple corer in an accident...

I normally go for as heavy as I can get and always look for rubber mounted ones, just me though.

The ones I'm getting are 450/500g I had some on my nexus and regret not putting the oem back on when I sold it.

Piaggio recommend the larger bar end weights for use with a top box... if stock standard use the same as the originals...

Bar end weights damp the natural harmonic resonance. It's a function of engine vibration and chassis and suspension syncopation.

You don't `choose` the weight. It's a result, not a choice.

You can see how a topbox might contribute or even invoke a sine wave resonance through aerodynamic or sympathetic vibration.

What he said.

You can think of the weight as an anchor held in place by its inertia - it doesn't want to move with the bars but once it does it doesn't want to stop again.
If the weight isn't rigidly attached to the bars because of a rubber mount or whatever - the push and pull works against the original movement.

"Harmonics" and "resonance" are to do with frequencies or the length/size/shape. If they match up mathematically as the same or a multiple they can dramatically increase the effect. The same as a park swing, if you swing your legs randomly nothing happens. If you time it just right on every swing, or second swing, or third swing.... you increase the swing. You have found a harmonic frequency for the length of the swing.

The bar weights are tuned to disrupt the frequencies

They put something similar in the top of skyscrapers so that they don't fall down in earthquakes.

They were the called "inertia dampers" before Star Trek nicked the name to mean something else