I've considered it, but never have for several reasons:
1) Used finish mowers can be found in my area for $500 or even a bit less.
2) Sourcing parts (gauge wheels, gearbox, etc) will certainly cost more than $500 making it non-cost effective.
3) Because I'm not an engineer, I default to overkill when it comes to building things. Odds are I would wind up building something that was too heavy for my tractor to effectively use.
Not to say it couldn't be done, because it most certainly could be... but I would have a hard time justifying it.