I answered a similar question a while back about the legendary 14th century English mercenary John Hawkwood. The short answer is that quite a bit of it would be "liquid assets," but these weren't carried around everywhere. They would be stored in safe places that a mercenary successful enough to have such wealth would own. They would also spend money to make money: some of the cash would go towards military costs (which were expensive) and some of it would be invested into businesses or used to purchase shares of public debt. However, a great deal of Hawkwood's wealth was property and therefore would not be portable at all. Hawkwood is an extreme example though, on the absolute upper end of the scale. Most mercenaries probably would only be able to dream of such wealth and status.