There are stock traders with Cesium atomic standards in their networks because high frequency trading requires such a level of accuracy that nothing less is sufficient.
Hell, there are two atomic frequency standards in my home workshop, precisely because telcos use them in almost every cell base station and they replace them regularly, selling the old gear off for scrap which eventually makes its way onto eBay for cheap, you can pick one up for a little over £100, Cesium standards occasionally turn up from the same telcos and I've seen them sell for a shade over £300 at the cheapest.
There have even been Hydrogen MASERs sold off and bought by hobbyists, rich hobbyists admittedly because they're expensive to run, you need hydrogen rated plumbing, a lab grade supply of hydrogen, a lot of power, a temperature and pressure stable room which takes a lot of space and backup power because (like all frequency standards) they take time to come up to temperature and long term stability so you don't want to have to switch them off and on again too often.
But, if you want a cheap frequency/time reference that's potentially as accurate as a Cesium reference then you can't get much better value than a ten quid GPS receiver with an outdoor antenna.
Atomic 'clock' is a pop science term used to describe an atomic frequncy standard, electrons don't know when it's pub 'o' clock (more fool them) , they just have a very precise energy state transition period which can be used to measure time.