Thanks to buying Tamiya's 1/350th Enterprise at full price about 11 years ago, starting it, losing interest in it when it was about 40% complete & putting it in the loft where it has spent the last 8 years waiting for the enthusiasm to return, I now have a price limit for a single kit.
Other than breaking it once for Trumpeter's Hood, I have limited myself to spending no more than a third of the Enterprise price on a single kit. As somebody said, there are always special cases.
Accessories can be painful with ship kits. PE can sometimes cost as much again as the kit itself. Also, although I managed to pick up Trumpeter's 1/350th USS Essex for half its usual price, I might end up spending as much again increasing the size of its Airwing. Buying another plane set every few months limits the pain a bit. Still, my choice to like ships, so no complaints
Good thing that I like aircraft kits too, which are a lot easier to find on discount.