Trade of Hard Assets is not the sole means of accumulating wealth.
Wakanda is technologically ascendent and would be able to achieve desired industrial or resource dependent outcomes at a fraction of the price and effort of most competition.
Tony Stark's revelation forges him into an altruistic capitalist, but a capitalist non-the-less. He gives as much of his resources away as possible to deserving causes, but the scale of this would be comparatively diminutive compared to a nation state. Wakanda's tech, whilst powered by Vibranium, could solve any number of problems regarding scarcity of resources.
Even before we consider the socio-political and medical advances their accumulative research and knowledge carries, from a business perspective alone; how much processing power do you think their technology is capable of? they may well be able to supersede all collective the Data-Farms in the world with ease, and onboard those contracts to within Wakanda, thus keeping all their resources and intellectual property.
Even while still political isolationists, this is one scenario in a million that could have already been at play before T'Challa 'opens their borders'. The value of Wakanda, and their co-operation, would immediately be attributed a high speculative monetary value.
The situation is Akin to an invasion of benevolent aliens on Earth; wherein they have arrived from outer-space with new technology and knowledge, but are not accommodated because they lack a trade currency; they wouldn't be expected to have any, but it would not see them cast out!