Depends who the Mekboy starts testing his invention on. I recommend that group of 'umies that thought they were smarter than him.
Since there is no hard-and-fast rule it is also the job of any GM to look briefly confused, and then explain that since the Rules As Written don't cover this the plan your players came up with didn't work but don't worry, you have an idea that will let them go through with this plan and then make them go through something excrutiatingly difficult as punishment for trying to sequence break.
Or we can try to be Scientific about this (I think that's a heresy)
A good squig pie and a tankard of fungus beer equates to a mid-grade meal, which is of Common rarity.
A "decent" weapon like a Shoota equates to an Autogun, which has a of Average rarity.
A warbuggy is probably one step rarer than a Warbike, which is Scarce rarity so we'll put it at Rare rarity. Vehicles are automatically more difficult to acquire, so let's make it Very Rare.
Now that we have some data point we can try to fil in the rest of the rarities.
Common - 1 Toof
Average - 2 Teef
Scarce - 5 Teef
Rare - 10 Teef
Very Rare - 20 Teef
Extremely Rare - 50 Teef
Near Unique - 100 Teef
Unique - 200 Teef
Now you just have to set the rarity for the item you want from the chart and translate into this (with a bit of haggling thrown in).
Power Klaws are Near Unique according to the Koronus Bestiary, so those will be 100 Teef.
High Quality Ramshackle 'Eavy Armour would be presumably be the equivalent of Best Quality for Orks (Buh?) and thus from Rare to Extremely Rare, or 50 Teef.
Kustom Blasta would involve taking a Blasta, and paying a Mekboy to "build" the upgrades as given in Into the Storm.
See if this works as a starting point.