What the market will pay for it. Just like anything else. And the trust actually does own physical gold, by the way. The fund strikes a NAV everyday, just like any other ETF or mutual fund, that is driven by the price of the underlying assets.
It's just that its shareholders don't have physical redemption rights, which is why gold bugs don't like it. Nor should they, if what they are really after is the ability to possess the physical gold. GLD is paper gold (you're buying shares in a trust, and the trust owns the physical gold). If I wanted to hold the physical gold, not just get exposure in a 401(k) to gold, I wouldn't own GLD either. If I own SPY, I can't "redeem" my shares in it for shares in each S&P 500 stock either. It's not how ETFs work.
Is GLD Really As Good As Gold?