In a Honda, the flashing D4 light is the equivalent to a "check engine" light for your transmission, so if a problem is detected with the transmission, it flashes the D4 light at you.
Too bad they say it's the ECU. That's definitely not a common failure for a Honda.
If you're so inclined, you can probably save quite a bit by installing the part yourself. It's most likely a matter of unplugging and replugging a harness or two and turning a couple of bolts that secure the unit itself (just like a motherboard). Kindly explain that you don't have $830 to give them (even if you do, who cares?), and ask them to point out specifically where the ECU is located.
I would either buy a new part at a dealership, or chance a used part from a wreck (you'll have to do a lot of calling around). Just don't buy one from an auto parts store. You never know what you might be getting. Make specific arrangements in advance with a manager so that if you buy a new one and the problem isn't solved (it turns out to be transmission trouble that the computer is detecting), you can return it. Many types of parts dealers have rather harsh policies about returns on electronic parts because of theft by dishonest customers.