I probably buy in pretty much the same circumstances as you do, and in about the same proportion to what I tie (95%). It's probably been a couple of years since I've caught a fish on a store bought fly, but that's more because I haven't fished anywhere but familiar trout streams for which I'm well supplied.
Like you, I will buy a bass popper or slider, but only if it's got a fancy paint job; I prefer to tie one with only one color.
Again, I'll buy on a trip, both because there are unexpected bugs on the water, or to get a model for future reference, or because I feel that if the shop keeper has given me some good info I should buy something. Unlike you, I don't usually carry tying equipment on a trip. If I'm going by plane, that would be never, and even if I do bring some tools along on a trip where I'm driving, I can't possibly carry every material that I own with me.
There are a couple of patterns that, while I can tie, I absolutely don't like to. Top of that list is a Humpy. I use them, can tie a reasonable one, and so hate the process that I'd rather buy.