True, but it is unlikely that you could build one that is cheaper than a commercial miniature rubidium clock.

Already a rubidium vapor cell costs about two thirds of the cost of a commercial Rb clock.

The Rb lamp is as simple as anything can possibly be, at least if you're not tasked with coming up with the right gas recipe.

Lasers, on the other hand, have to be frequency-stabilized for use in high-quality Rb clocks. Every time someone has tried to use an unstabilized laser to replace an Rb lamp, the results have been worse in one way or another.

Usually stabilization involves a separate loop with an Rb gas cell. A bit of an oversimplification if you count miniaturized Rb standards based on VCSELs, but those are at the very bottom end of the performance spectrum to begin with. As adrian_b suggests, the best crystal oscillators are superior in many respects to the worst rubidium standards.

So a good laser-based standard will never be more economical than a lamp-based standard unless someone finds a way to run the laser open-loop. AFAIK that hasn't happened yet, at least not with something I can order off the shelf from Edmund or Thor Labs.