Indeed, there isn’t a low cost DVL yet. There are a few reasons: cost of components like high end ADCs/FPGAs, multiple potted transducers (think 4x the Ping sonar just for the head), but also the fact someone had to hire a lot of costly experts to develope a product with a steep learning curve and a small customer base.
All that said, I think it is feasible to get out of the $15k+ range and into the $5k range. Maybe lower if it’s a more analog based system and leave “bottom lock” into the programmer integrating it with the INS data.
Getting to your immediate need, Craig is right. If you need absolute position, it’s probably WaterLinked SBL or something like a Micron USBL coupled with your INS. If your flight path can afford it, surface and re-zero your AUV using a GPS antenna just above the water. If you have the budget, a current meter can help you INS.
I have flow work class ROVs for a decade and DVLs are handy, but they are only so accurate and are generally used to “filter” USBL data in conjunction with an INS. If you loose bottom lock in soft silt or a school of fish fly under the AUV, you can loose 10 meters in a moment and not even know it. That’s where INS and knowing you absolute position (via GPS and LBL/SBL/USBL) come into play.