Define cheap, lol. The biggest issue with a dyno is the flywheel and enclosure. The electronics part isn't too difficult, but sourcing a flywheel... Well, there is the software part to, lol. But connecting the mcu to a usb and using a terminal emulator you can get the raw output and use your favorite spread sheet application to come up with something.

An inertia dyno is on my list though, not sure how long it will take to get to it.

Designing a timer to start and report info is easy. Determining max RPM on the other hand is all about how accurate you want it. Most likely the max rpm will oscillate a bit, so you need to decide how to determine the max value.