Does the antenna you bought show a squashed donut shape for its
pattern? Does it show a pattern at all? Telling you 9dBi doesn't say
very much on how the antenna performs. It may be (unlikely though)
that the 9dBi is only good through a few degrees of space (a pencil
beam pattern). Knowing something about antennas obviously helps.

You posed a good question. It made me do a little research and I came
up with the following. As you may also recall, if you space your
elements in a an array lambda/2 apart. You get higher gain in the
broadside direction. Say you stacked many monopoles (end to end) and
made sure their phase centers (feed points in many cases) were lambda/2
apart the donut shape would be slightly preserved, squashing it down to
a flatter donut, thereby increasing the gain. (Higher gain equates to
narrower beamwidth). You would ultimately end up with a donut with
ripples as you moved from the "hole" to the circumference of the donut.
The points farthest away from the center would be the 9dBi gain. And
as you would expect, you would only get that 9 dBi through a few
degrees from the center of the antenna. The tricky part in this is
feeding the many monopoles inside the plastic shaft in which your
antenna array is encased. You want to be able to feed the monopoles
without having your feed network radiate and throw off your desired
pattern. This is where smarter people than I come in...

As for your question on the price... As Bob Bob said, gain isn't the
only characteristic antennas have. The price difference could
represent a differene in phase linearity from one point on the pattern
to another. The matching network, beamwidth, bandwidth, side lobe
levels, and a few others also make one antenna more desirable than
others.

Regards,

Dario