By weight (I'm not finding handy data in terms of value) that's about right but a little on the low-end.
According to Statista, 52% of air cargo by weight was transported in the bellies of passenger planes in 2016.
The IATA (see page 17) says that about 28M tonnes of air freight was carried by passenger planes, or about 45% of world total, in 2018.
If that seems surprising, keep in mind that the vast majority of freight is moved by surface, not air. Air is expensive and only used when the greatest possible speed is required. That applies to passengers and high-value perishable goods, but not most items that make up the bulk of international trade.