I'd say it depends on many factors. Development costs may be higher or smaller based on the country you're developing the platform in. For example, I'd say developing a Coinbase-like platform in my country (Romania) or India would require much less money than it would in Sweden or the USA.

You could probably work with Romanians or Indians from any country on such a platform too in order to minimize your costs, but I would not trust working over-the-air with some company on a serious business. Face-to-face will always be better when it comes to negotiations and business.

The best thing you can do imo is create a well-made list of features you'd like implemented and start calling or emailing different reputable companies and asking them for an approximate price (I guess it'd be +/-$10-15k) of a platform. They will probably also give you a deadline.