I'm currently in the process of searching for a cheap standalone ecu, but the main limitation of them is the fact that they have only 4 injector outputs. I have heard and read in the ecus documentation that you can pair two injectors together and therefore use three injector outputs for a six cylinder engine and four for a eight cylinder engine. The thing that boggles me is that a solution like this would be inefficient(?). From my understanding let's say in a V6 you pair two injectors wich pistons are at TDC, but they are at different cycles. Only one of them is firing(?). Could I use this for a daily? And what about the ignition?
Yes, paired injectors is one solution, but have you looked for standalone ecu's with more outputs? It does affect the fuel use obviously, as only one cylinder is firing while the other is closing exhaust and opening inlet.
As for ignition, the same concept applies - double ended coils will work as a spark into a cylinder changing from exhaust to inlet is not an issue.
As an alternative, depending on how cheap a cheap ECU is, you could use the Ferrari method. On the Ferrari V8, they run two ECU's fed from the same crank position sensor. One runs one bank and the other, the other. Of course for this to be cost effective, the difference in price between a 4-cyl ECU and an 8-cyl ECU would have to be significant.
You should define what you mean by "cheap" because what you think is pricey might be cheap to someone else. Even an ECU meant primarily for 4-bangers can be hacked to work with a V6 using wasted spark and 2 squirts per cycle, e.g. Speeduino, one of the cheapest standalone ECUs I know of.