Check the amp rating on the back and the amp rating on your light.
If your light draws say, 10a, then you need a timer rated for 12 or 15. Also, never try to run a grounded fixture off a non-grounded timer.
A grounded 'heavy duty' 15a timer should run you 10-15 bucks. A timeclock that you can plug 2k of lights and 1k of air handling gear into will run about $40-60 and require an electrician to wire.
In between, you can find random timers especially at this time of year. I like the intermatic 'outdoor heavy duty timer' for growroom use. It is perfect for a mid-size lamp and blower, and has a cover to prevent water splashing on it.