My personal rule is to consider how much else I could buy instead of that item (or what I am sacrificing to have the item in question). For my personal income level for example, I have to think if I buy a bag or jewelry piece, is it something that I can wear while taking my kids on vacation, or was that the vacation budget I just spent? lol
With regards to net worth, are you including your retirement savings in that figure? The equity on your house? It is an interesting way to look at it. I'm not sure it would necessarily be a good indicator of whether or not you could really afford the piece. Seems like cash flow and overall financial health would be a better indicator. I would say that if you had proper retirement savings going (and you contribute every month if you're still working), proper emergency savings, etc., AND you can easily pay your bills every month, plus have extra for fun stuff, then I don't see why you couldn't splurge on yourself just this once.
For example, if your net worth was $1M, then you are only talking about spending $10K on a piece. But if that net worth is mostly in assets that you can't really touch, like your house and retirement, then you may not have enough in your checking and savings accounts to make it a safe purchase. Again, using myself as an example, I have a certain amount in home equity and retirement savings, but we put so much into various savings funds each month that our cash flow is limited. So, using a net worth percentage would make it look like I could afford more than I really could.
Also, I'm also not sure if you are using the down payment on an investment property as an example of something that is in the same price range, or if you are saying that an investment property is something you are genuinely considering. If you are considering an investment property, one option would be to get the property and use a portion of the profits to save into a "special jewelry" fund and get that special piece later.
Whether someone spends a good chunk of their money on something that will make more money, or give them immediate pleasure is a very personal choice. Realistically, many people are probably spending way more than 1% of their net worth on luxury goods and services. Is this coming at the cost of setting up a safe and secure future? That's something each person has to answer for themselves as circumstances vary.
Best of luck with whatever you choose to do!