Symbols, runes, writing. Anything not mechanically useful.
I would say included in the cost of masterwork. I do not believe there is any RAW costs for beautification of weapons
Symbols, runes, writing. Anything not mechanically useful.
As much as you want to pay. Look at tattoo for a base line.
Tattoo, Price 1 cp–20 gp.
"The price of a tattoo depends on the quality, size, and number of colors used. A coin-sized tattoo in blue ink that will blur over a decade costs 1 cp, a hand-sized one in black ink that won’t fade costs 1 sp, and a tattoo covering an entire back takes several sessions and costs 10 gp. Each additional color costs as much as a single tattoo of its size. While these prices represent the baseline, tattoos—like any other artwork—can be worth far more depending on the prestige of the artist."
SO pretty much 1cp to as much gp as you have.
Agreed with J4RH34D. It's pretty much just flavor, so lump it into the masterwork cost and add it to the description of the item.
I think Graystone and J4RH34D have it between them. For a masterwork it is included in the cost. For a standard weapon, 1CP to 20GP seems reasonable. Alternatively, if precious metals and the like were being used then craft as a piece of jewellery after crafting the weapon.
I don't know that I'd agree with throwing it with mastercrafting. Mastercraft is all about function: it's SO well made, it gives you bonuses to use it. Fitting perfectly in your hand has nothing to do with looks. You might have the UGLIEST weapon in the world, it just works really well.
So I'd say cosmetics are extra.
You could scratch your initials onto a sword with a nail picked up off the road. Some people did. At the other extreme a king might pay a ridiculous amount for something to prove that he was able to do so, to appear generous and to reward a supporter. Zero to ridiculous amounts.
Aside from looking up prices of artworks similar to what you want I can't think of any good guidelines.
Ultimate Equipment's appendix gives a "bejeweled sword of state" a value of 6,000 gp. That should make a good upper bound.
Look at images from the 'Staffordshire Hoard' a large part of it is anglo saxon sword decorations. Having seen it in person the craftsmanship is breathtaking, and how skilled the craftsman was to do some of it without the benefit of magnification lenses is awe inspiring.