It is hard to meet if you're starting with D or E resources which my mages normally do.
If you start with resources D or E as an awakened you are probably more or less living on the street as a SINLess individual anyway... Just don't showoff your magic powers in public... and don't get caught. Or trade some of that starting karma for resources (but I can agree that the prices for fake SINs are a bit high, at least for the lower rated).
If you prioritize resources higher you probably have a medium or even high life style - and the need for a fake "legal" SIN to go with it is higher. Samurais might for example spend some resources to have licenses to possess firearms and license for concealed carry tied to a few different (rating 1-3) fake SINs (if the SIN is burned the licenses tied to that SIN are also burned), a "legal" fake SIN (rating 2-4) and maybe a "clean" and unused getaway fake SIN (2-3). Most Face would probably spend quite a lot of resources on various high rating (3-4+) fake SINs and separate wardrobes to fit each alter ego.
Get a R6 when you retire from the shadows and want to live a "regular" life.
Your "legal" (medium or even high lifestyle) SIN is a SIN you can "keep" and build on. Just need to keep it separate from your illegal activities. But as you use it over and over odds are that eventually it will get caught in a SIN check and get burned. Your "legal" alternative life should probably have a high SIN. Eventually when you got enough resources to spend this persona should probably ideally be a R6 fake SIN.
As you use your "criminal" fake SIN alter ego it might get implicated into various crimes or crime scenes. It might also been used as a cover when setting up some influential individual. Keep using the same SIN over and over will eventually make you easier to track. Reason alone to ditch the fake SIN even if it is not yet burned. Some like to keep a low profile, hiding in the shadows and use a high rating fake SIN for this while others use several lower rated fake SINs that they can alternate between and ditch before they get too "hot".
Game mechanic wise what is "best" and "good enough" will depend a lot on what type of GM you have.