Perhaps you should have suspected something was amiss when your agent started humming, "Lemon tree very pretty," as she flitted about the backyard photographing fruit for online posterity. It's obvious your relationship with her has, forgive me, "soured," and we'll get back to that.
As for your initial question, yes, you certainly can list your home on a FSBO site while simultaneously employing an agent. But the time to make that arrangement was before signing your listing agreement.
In fact, depending on what type of contract you signed, you might be on the hook for more than your $300 to $500 guesstimate, particularly in an "exclusive listing" sale, where the listing agent pockets the commission regardless of who produces the buyer. In an "exclusive agency" agreement, however, you retain the right to sell your home without paying commission as long as the buyer didn't come through the agent or agency. An "open listing," which you can have with many agents simultaneously, basically states you'll pay commission only to the agent/agency that produces a buyer. The latter two options, I might point out, tend to be demotivators for busy agents.
If your present agent agreement is about to expire, you can just wait it out, then hire a new agent choosing one of the latter two listing options. But I sense you're in the early stages of at least a six-month listing.
Did you discuss your ongoing concerns with the agent yet? If she knows you're unhappy, she may alter her strategy and recast the MLS listing based on your wishes or agree to write up a different contract giving you FSBO latitude -- or even pull out entirely. Most agents don't want to waste their time where they're not wanted, especially in a robust market where there are many other opportunities.
Ostensibly, you might think using both a listing agent and FSBO listing doubles your chances of selling. But know that FSBO listings tend to attract a different breed of buyers expecting to pay less because sellers are saving on real estate fees. Plus, sellers have no one to walk you through the myriad paperwork involved in a transaction, unless they pay a set fee to an agent to do this (some agents will).
Real estate agents do incur more expenses than you might think, including at least a portion of those yard signs. There are MLS and real estate association dues, mailings, fliers, Internet expenses, business cellphones, cameras, car expenses, business cards and sometimes print ads, not to mention "showing clothes" and other work-related expenses, particularly since so many brokerage agents are actually independent contractors.
Anyway, here's hoping you were able to minimize the note you were typing to me before she spied it and that your party finds a way to maximize your home's marketing potential. Good luck!