Accuracy is a must to me no matter what the cost. I was upset with AMT because they put the wrong bumper in their 1966 Oldsmobile 4-4-2 W30 kit and the bumper is a big part of the W30 package even though it's a really nice kit otherwise. Making it right should always be job one.
After that, fit and/or detail relative to cost is crucial. If I'm going to pay Tamiya costs, I DAMN WELL expect the kit to go together as well as a Tamiya. Or, for that matter, Fujimi Enthusiast series kit prices should equal Enthusiast Series detail.
Finally, subject matter is important, but not critical. There are many cars I love that there are kits of that I will not buy because they aren't worth the cost. If I hit the lotto, everything on my shelf would be Studio 27, Fisher Model Works and Pattern, Modelhaus, RMCoM, and Le Mans Miniatures. But, until that happens, I can't always justify $70 for a kit so it had better be one spectacular kit at that price for me to buy it.