If price is the question there are rowbikes offered by the manufacturer and sometimes from individuals.
Although I like it, and needed the upper body workout:
1) It can be hard to keep you lane while rowing so I think it is best if you live near a bike path or a wide quiet road.
2) If you do not use both hands on the lever the bike might go out of control.
3) Low clearance between ground and lever, On broken pavement I have scraped the ground.
4) 20" wheels and no suspension mean a rocky ride.
Otherwise it does work as expected, I can even do hills, thus far the only mods I have made was to replace hex nuts with wing nuts so I could get the seat and foot rests off easily for storage.
For the record I am not selling any rowbike(s).