Excel is not comparable to CO2. It is just a different source of carbon that plants can use, but in no way is it equivalent to keeping 30 ppm of CO2 in the water.
DIY CO2 is cheap only in that you never spend a lot at one time, but for a big tank you do spend a lot for sugar over a year. It is not easily controllable, so you can easily get too much in a small tank, you can't easily shut it off at night - you can just waste it at night. And, it is very, very labor intensive.
Pressurized CO2 will not "fail" if you install it reasonably correctly. It just keeps on bubbling CO2 into the water day after day after day, at the same rate, and is easily shut off at night and turned on during the day. Once it is set up there is almost no labor involved with it until you need to refill the CO2 bottle, which will usually be after at least three months.