Hi @rezenders, welcome to the forum 
This thread is a good starting point for understanding the various available techniques for underwater positioning. The rough relative costs in the main post haven’t been updated as new devices have come to market.
In terms of actual devices, WaterLinked’s Underwater GPS and DVL pages provide some useful information about the technologies, and the DVL page includes a comparison with some other DVLs and GPS units. Beyond those mentioned there I’m also aware of Cerulean’s ROVL mkII and their DVL.