Itll take a lot of time and effort to build your own speedos. Im working on a fether speedo as well. Right now, the schematic is almost done. Meaning I still have to work out some details, this can take up most of your time. Im hoping it will fit the size of the PCB I usually use (half a eurocard). The layout stage wil probably take a long time aswell.
In the end, I may see the first versions go up in smoke if I made a mistake. Im keeping my fingers crossed.
So its probably not cheaper, because youll probably blow up the fist few versions and it will take a long time to get it working properly.
The main hrudles are getting the timing of the fets right and current limiting.