Hi @moro, welcome to the forum 
I would note that if this was something readily available it would almost certainly come by default on most ROVs. That said, there’s a strong dependence here on your particular operating conditions, and what you consider to be sufficiently robust.
I’d suggest you read this topic to get started with understanding underwater positioning technologies. From there it may be relevant to consider the following:
The systems discussed in those two links are useful to know about but are also all outside your stated budget, so if it’s of interest, the lowest cost options I can think of right now are:
| system | cost | notes |
|---|---|---|
| IMU double-integration | Free (IMU already in ROV | loses significant accuracy within a few meters of motion → generally not feasible) |
| Standard GPS with antenna | can be ~$10 to a few hundred | only works at the surface, may be difficult to integrate well |
| Optical flow/computer vision | camera(s) can be ~$50 to several hundred | only works with good water quality and distinguishable objects, likely requires significant integration development and some potentially expensive processing hardware |
| Cerulean DVL-75 | from $1999 US | max operating height ~40m, max speed 8 knots (~4.1m/s), above your specified budget |