I worked on a different xenon-based experiment[1] for my PhD. We were looking for similar rare decays to learn about neutrinos. Our observation of double beta decay of xenon 136 used to be the record for rarest decay.

Xenon has some very nice properties as a particle detector for rare events:

1. For one thing, it's a noble element, and so it can be purified very well to reduce background decays of other elements.

2. It's also a very heavy atom, and so it's self-shielding from external radiation. The core of the detector is shielded very well from radiation coming from the outside, so any signal you see there is most likely from decays of xenon or from things like dark matter WIMPs that don't interact much with matter.

3. It is a natural scintillator[2]. It gives off light when an interaction or decay ionizes the xenon atoms. That lets you actually detect the event, and by collecting the scintillation light, and the electrons from the ionization, you can get a decent measurement of the energy of the event.

4. It's recyclable. The XENON1T experiment follows the XENON100 experiment. The 100 kg from XENON100 were reused in XENON1T, and the tonne from XENON1T will be reused in future experiments. So the cost gets amortized.

$120/g sounds on the expensive side. The price is always changing based on supply and demand. One manufacturer deciding to use xenon in some process, or finding a way to replace xenon with argon, can swing the price by an order of magnitude.

As for leaks, I can say on our experiment we took the possibility very seriously. The entire xenon gas system was made of ultra-high-vacuum plumbing, and we helium leak checked every connection. When the xenon was outside the experiment in bottles, we had sniffers around the bottles to make sure they weren't leaking. We also had emergency systems in place if we needed to recover the xenon, including a "balloon of last resort" that would've captured the xenon in the event of a catastrophic failure.

[1] https://www-project.slac.stanford.edu/exo/

[2] https://en.wikipedia.org/wiki/Scintillator