A difficult question to answer.
As has been said previously, some sets are of lesser quality. However a bit set from a known manufacturer may be a different case.
For example, a manufacturer offers a bit set and specifies bit numbers in the set. The quality of the bits in the set is of the same quality as what you would find if you purchased them individually. The bad news is that the cost savings is probably not much more that 10% or 15%.
That is only part of the issue.
A more important question to ask yourself, is "How often do you expect to use the bits from the set?" Are you buying router bits that will serve as dust catchers?
There is a router bit set floating about containing of over 60 bits for about $200. Let's work that price backward. On average that is $3 per bit. (Round numbers) The retailer buys the bits at about $1.50. There are some shipping costs, distributor to retailer. The distributor buys the bits at $1 and there are shipping costs from manufacturer to distributor.
Forgetting about shipping costs, the manufacturer's sale price is close to $1. The questions are:
Where in the world can a router bit be manufactured profitably for a buck?
What kind of quality can you expect in a $1 router bit?
Finally, think of the physical position of the router bit relative to your body when you are using the bit.
Now, do you want a router bit, that the manufacturer sold for about a dollar while making a profit, spinning at 20,000 to 30,000 RPM, in that physical position?
