Hey,
I think that there were similar threads before and you can try to search for them.
Anyway, you can create depth my by yourself using fishfinder and software (for example Dr.Depth -
http://www.drdepth.se/).
About fishfinder - you'll need not a simple toy but something more advanced with GPS on it.
Last year I bought Lowrance Mark-4x for fishing. It has GPS in it but it's not able to record depth data. Although it's possible when laptop is attached to it. If you got some chartplotter device then you will be able to work with it only.
So, the costs could be: Fishfinder + boat + software.
If you are able to borrow a boat and fishfinder for some time then everything that's left is software price ($ 140). If you steal it then it's almost for free
(I didn't count your time and similar stuff).
