OK, I see what happened. Stupidly, I hard-coded the recipe to what I found in the Base files, which is 10 of each per Part (so 10 of each per Kit), and I'd have probably hardcoded 100 Parts (100 Kits) per rocket. Sneakily, space-age does this:
data.raw["rocket-silo"]["rocket-silo"].rocket_parts_required = 50
data.raw.recipe["rocket-part"].ingredients =
{
{type = "item", name = "processing-unit", amount = 1},
{type = "item", name = "low-density-structure", amount = 1},
{type = "item", name = "rocket-fuel", amount = 1}
}
So it makes rockets a lot cheaper. Looks like I'll move Kit recipe creation to data-final-fixes and read costs off the vanilla recipe, instead. Probably also add Space Age as an optional dependency, just to ensure load order.
Well, stand by while I go do that :)