Cheapest - buy a few hacksaw blades. Wont take too long 30 mins or so
Quickest - angle grinder
if you have the time and energy use the hacksaw - if you hit a weak spot in the pipe with the angle grinder it can shatter, and may end up being a bigger job - less likely to happen with a hacksaw.
Using just a hacksaw blade (no saw handle) takes 30 mins.
Only kiddin, I'm a time served plumber but don't do much with toilets now - like to stay clean.
Been a few years since I cut a cast iron soil pipe - I read another post saying it took them 30 mins I couldn't remember it ever taking that long but I just relayed that info to tony636.
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