You won't find the top VO talent using USB mics for several reasons. The mic portion aren't as good as the good studio mics and the A/D conversion of the USB mics isn't really all that good. I have tested and reviewed the Centrance MicPort Pro USB adapter and found that it works well with my best mics but doesn't approach the sound of one of those mics with one of my better preamps and better AD conversion. In addition, plugging in a USB mic to a laptop without regard for the acoustical environment may not meet your client's quality requirements.
As a member of AFTRA since 1970 and a member of SAG since 1986, I welcome you to the professional performing unions but caution you about working for free. First, if you start working for free, you have already set your price. Second, as a member of SAG, you can not work for free. Please go to the SAG web site and read about Rule One.
The idea of purposely working for free to get yourself into the market, while a staple in a capitalistic system, is anathema to SAG rules. As a member of a union, you are a member of a professional organization; one of whose missions is to establish rates for its members that will allow them to make a living. Working for free obviously doesn't do that. Lowballing the rates of other members is diametrically in opposition to the one for all, all for one concept of unionism.
I don't know which chapter of SAG you joined, as you live in Charlottesville, VA, but if they failed to explain this to you when you registered, they were not doing their job.
Please also know that while your posting at Voice123 indicates that you will accept PayPal for payment, SAG jobs can not be paid by PayPal. You need a paymaster or to be paid by a signatory who will put you on their books as an employee. Not many want to do that. That's one of the many reasons paymasters exist.
In general, Voice123 is for non-union talent who can hammer each other until the inquiring producer gets the low price he or she wants. Again, not a great way to make a full-time living as a voice talent.