I'm always amazed when I see someone using a 5mm hex key that they got from an Ikia furniture build kit on a $6000 bike.
Here's what you get when you use a cheaply made tool:
1) Poor tolerances. For example, your 5mm hex key could be 4.95 mm, which is not good.
2) Poor quality steel. For example, your 5mm key will very soon be a 5mm round thing, similar in form to a 5mm key but prone to ruining bolts.
So, you want 'high quality' tools -- high quality steel and tight tolerances. In general that means made in Germany, Japan, Italy, or the US. This is not to say that you can't buy high quality tools made in China, but a $20 set is probably not a good bet. I'd say $10-20 *each* is more like it (a good 10mm hex key can be $35, while a good 2mm key might be $10). Don't believe me? Look up Beta tools.
Simply put, to avoid damaging your very expensive stuff, you buy the best tools you can afford. Any mechanic will tell you this.
These are excellent quality, and not particularly expensive for what they are, especially considering the extras.
Bottom line: These are very nice tools, at a moderate price point. You could buy some cheaper, but as a rule, if a set of 8 hex keys doesn't retail for $75+, then it's probably not high quality. If you want really nice stuff, this is the entry level.
Note that a smart shopper doesn't pay retail, but then these retail for $125. In my estimation the current asking price of $98 is a steal.