Just got 2nd Age of Walkers and I was wondering if ther was a specific formula you were using for magic items used on mech to determine cost, XP, and PU. I noticed the 2 rings built into Tanniel and wanted to know about doing similar things for my designs (I'm thinking of maybe Boots of Striding and Springing for a scout mech).
Excellent idea for a scout mech -- although I imagine some coglayer somewhere is cobbbling together Murgatroyd's Mega-Speed Gear Relays
to prove the primacy of steam over spells. Anyway, I built Tannanliel, and since it was a huge and unique mech I left the underlying magic-item math to GM's discretion. From a broader standpoint, here's my thinking...
Magic items for a mech are like magic items for a person, only much larger. This means they cost more; if you're designing an item for a Gargantuan mech, look at the cost for a normal longsword and compare it to the cost of a Gargantuan blade. Whatever that cost multiplier is, it applies to your new item.
PU use is again based on what it would be for a person. If it's an item that takes up more space than clothing/jewelry on a human body, it takes up PU on the mech. How many PU? Depends on the size of the item and of the mech size(s) it's intended for. We've scattered a few of these among the books.
XP cost is something I base on size. To be frank, I've never had PCs design magic items for a mech, so I haven't got a hard-and-fast rule on it. My preference is to assume that every increase of size doubles the amount of XP required, but that multiple builders can use the Combine Spell feat to jointly contribute XP.
How's that work for you?