Ona can cover up smells of small grows that are pretty contained...But for larger grows in more open environments, you'll need a combo of carbon filter and Ona to do the job.
I can understand not wanting to shell out big bucks for an industrial Carbon filter (CarbonAire, etc) But theres cheaper alternatives...you can make your own carbon filter (lots of DIY-Carbon Filter threads and videos)....But theres also a product called Odor-Sok. Its a cheaper version of a carbon filter, and utilizes carbon-impregnated fibers. Theyre like $40, and work well with most small-medium sized grows. But even then id use Ona with them.
The ONA block lasts longer, but is less intense...The gel can be utilized in different environments...and it can even be spread on things and areas (if you're so inclined to spread gel on your grow area) but be careful as to not put any ONA near your plants, or in your grow area, as they can affect smell/taste of your final product.