@teckel - Those (HC-SR04 ) are what I am using in my project too (this post).
If you get them as "one unit" on amazon, they can be $7-10. If you get them in a science lab pack of 5-10 units, they can be as low as a few bucks.
It seems like there are a few nice benefits to the newping library, and looks like @ScruffR and others ported it to Particle:
(but from that post, not sure if it's actually working as expected)
I'll check it out.