I can imagine the situation where you would want an expert, but for me that would be more when I'm trying to fix a specific issue (users don't convert well on page X), and I don't have a specific problem. Right now I mainly want feedback to validate my core concept, and really that needs a certain volume of opinions.
OP, I'd suggest splitting the pricing: $X for an expert opinion (I think you could go quite a way above the current $5 here), or $Y for N opinions from random people who might fit my niche (you'd need some broad categories somehow though).
I will think in your idea of splitting the price. The true is that "in a way" I did (you can buy specific experts) but some people say to me that the price is too low... But it's hard for me to upraise the price without any sell :S
Thanks!