My question is what lang do they want this built in. Depending on the language depends on the cost of development. A p2p exchange is no issue but what you want it made on could more or less tell us if you can afford a team or not for the amount you have stated.
Also watch out for people who will offer cheap prices most the time they are not up to date with the current forms of libs or up to date with the programming languages you want.
The developer can choose the language to build it in!