S-band and beyond will cost you more, but a fully-featured VHF/UHF/S-band one from a proper vendor will be under $100K.
X-band, yeah... But not that many cubesats have X-band transceivers anyway.
What the Amazon website doesn't cover is:
- download guarantee. They rent you the antenna, they don't commit to a successful reception (and consequent refund if it fails)
- link status report (needed for a positive confirmation of successful reception.) This is basically a series of SNR measurements
- competition with other users of the same antenna: first-come first served? Maybe your spacecraft needs more storage to save the downlink data if the Amazon antenna is busy.
And of course they don't talk about the competition.
And it's normally better to just have more redundancy instead of higher individual uptime, at least as long as space, weight and power are not an issue (i.e., on the ground).