The world's largest bitcoin fund is trading at 73 cents on the dollar. That may look like a bargain if you want crypto in your portfolio. But nothing is really free on Wall Street, including a cut-rate deal on bitcoin.

Holding $26.2bn in assets, the Grayscale Bitcoin Trust owns more of the cryptocurrency than any other fund. A private-placement trust that trades like a stock, GBT has become a popular vehicle to access bitcoin in equity form. Institutional investors like Morgan Stanley and ARK Invest hold it in funds for clients. And it looks cheap, trading at a 27% discount to its net asset value, or NAV. If the share price of GBT matched its NAV, investors would notch a roughly 37% gain, even without a nudge from bitcoin.