bro, deffinitly talk to her. you need to come to an agreement somewhere. it's only a hobby and the money spent on it should be money that is not attachted to any house hold items.
there has to be a middle ground. IE, if there is 100 bucks a month of "free money" on your behalf, then that could be your budget.
with my wife, it's like most on here. theres a roof over their head, food in the fridge, bills paid and cars maintained, well then theres not problem with buying something.
or trade kits, sell kits to buy more. i sell 1:1 car parts that i have lying around to buy most of my stuff. i also have traded kits for kits. shoot, im currently trading the clear parts from a skyline r32 for a e55 amg kit that has been partially built, but it's complete. what am i gonna do with it?? dunno maybe trade it or keep it..it's in the air.
dont go behind her back to get stuff you want. reason with her, it's easier. she'll feel as if your cheating her by doing so.