Analysis by Sigil fund states that with current aggregate dex volumes of around 1M daily, 0x is around 50-75x over-valued using DCF (Discounted cash flow) model. Source
However, DCF model doesn't take into account expectations for the future. With the advent of DeFi, it's not unrealistic to say that DaPPs using 0x protocol may reach 1b and beyond (current Binance levels) , which equates to around $5 per 0x under the same DCF models. Coupled with a future expectation & block-chain multiplier, $10 - $30 is within reasonable bounds.