For the stuff I make, I don't charge more than 3 digits, except for a custom Wasp Prison from the Spider Garden series that someone happily paid me more than I think it was worth.
If I make something, I look at other similar things inworld, compare prices and price it somewhere in the median. Not too expensive so as to make it out of reach for most people, and not too cheap that everyone and their mom will be wearing it. Prices are usually 299-799
As for shopping.. well, I hate going to the Bare Rose sim, because by the time I am thru, I usually would've spent 1-2k without knowing it because it's like "oooh pretty! click, buy." "ooh,, this one looks nice. buy." "ohh... nice hat! buy whole outfit." For dresses, my limit is still three digits. L999 at the most, and it better be copy/mod, or the creator is willing to give you a copy/mod version. Skins, the most I will pay is 800. (yay nomine!) Hair, 350 at the most.
For my other guilty pleasures, full body avatars, I usually will pay upwards of 2000 if it looks really, really good, or if it has unique features like being able to breath fire/ice/rainbows or if it's something i have not seen on anyone else before. The most expensive this I've ever bought was a 3500 dragon, and I wanted it so bad I saved up for 3 whole months for it back when i was new.