Edit: How do you put pictures in posts?
Here's a picture of a map I made for my Garmin.
Cheapest? Do it yourself. I have all my lake maps on my Garmin eTrex 20.
First download QGIS and Google Earth Pro, then download the free Government of Alberta Lake Bathymetry data for whatever lake you want.
Download the .zip with line features (not polygon features) and import the .shp file in QGIS. Right click on your layer and open your attribute table, then count how many features (different depths - 1 line is 1 same depth) you have. For example, the lake has 10 seperate lines ranging from 1m to 10m deep, so you have 10 seperate line features. (all gov. data is in metric by the way).
Save a copy of your shapefile and import it to Google Earth Pro. Add a template with a colour field (I prefer blue and red - blue is deep/red is shallow), and set your number of buckets to match the same number of line features in your attribute table. Once you have your map and template in Google Earth take a screenshoot that encompasses all of it and save it as a JPEG.
Then you just follow these instructions.
http://www8.garmin.com/outdoor/custommaps_instruct.pdf
It's pretty easy to do a JPEG overlay on Google Earth Pro with a screenshot you took from the same exact spot. To make it easier on yourself make sure you're oriented North and looking from right above and not on an angle when taking your screenshot. If you don't do this your picture will be all distorted when you try and overlay it. I also drop pins on my corners to make it easier to overlay later.
Here's the Alberta GIS data for most lakes.
http://ags.aer.ca/alberta-lake-bathymetry-data