I approach this differently.
If I can't get a good quality recording with an SM58, I have problems that are unrelated to the mic, instead, performer, performance, room, MONITORING, cables, software, FX, or even, OTHER tracks with problems.
At the performance level, I think MONITORING (with cans) is perhaps the most important, and under appreciated, piece of the pie. No other thing impacts the instantaneous feedback loop that governs the feelings and emotion of the performer more, and yet, most jam cheap cans in available headphone jacks, and even those who take time with their monitor setups often skimp with mid or consumer quality gear. A stellar performance (achieved by putting the performer WHERE HE NEEDS TO BE by attention to STELLAR monitori setup) on pro-sumer capture and recording gear will outperform a poor performance on million dollar mics and gear. I believe that MONITORING will become the next major focus thru the 2010 decade, much as room treatment has been since 2000, and in my mind it is of more importance. After all you CAN get a good mix in a poor room, but you can NEVER fix a lackluster, who gives a damn, clip, in a great room.
Once I have it where it needs to be, I audition a couple other mics, namely an AKG 200, and then a Neumann 102. These run thru a Focusrite ISA 2, a solid high-middle quality pre. I take most of the room out of the equation with gobos and a stand mount baffle.
That's a start with a legendary, and forever repeatable $100 mic, progressing to a roughly $200 mic, ending with a $700 mic, all of which are solid dependable performers at their price level. I can throw four and even five figures at "mic problems" which really are NOT mic problems, but instead I cut to the chase and get those other problems fixed with a standard or wireless 58. There are times when I audition better mics, and record with the 58, because it works better, and times I never audition the better mics because the 58 catches all there is. The better mic options are always there if I need them. Until you are booking major market projects, most of your customers who know the difference between a 58 and an 87 are the folks who heard that ProTools is "better" than Sonar. Educate them if you can, pass on them if it means suicidal business practice or philosophical selling out, and for the very few who can make use of high end equipment, discuss their budget and rent the best they can afford. This formula should work equally well with other, similar price/performance components.
This approach ALWAYS gets me to the "Near Stellar" level. Since my facility is not fully live, still in config and fine tuning, this is MY most efficient way of making the recordings **I** need for now. Until my revenue streams solidify at the major label/album project or comparable level, sustainably, it is simply self delusional to succumb to Gear Acquisition Syndrome, along the lines of sows ears and silk purses.
As the business end progresses, I intend to RENT higher quality equipment and learn the vagaries of that equipment in MY ROOMs, at customer expense, less any learning curve delays of significant nature.
At that point, I have two basic options. I can invest in my future BUYING selected and familiar equipment, that my revenue streams do not (yet) support in the hopes that they will, or I can BUY equipment that the business DOES support, and make a bit extra profit avoiding the rental fees.
If you have money to burn, just back a truck up to Guitar Center and clean 'em out.
If you don't, I submit that buying gear past the "Near Stellar" level is one of two things.
Blind self delusion, which is probably masking OTHER OBSTACLES as yet unaddressed, or a disciplined, occasional reward to yourself and natural human wont, which I am totally kewl with, IF you are!