I wouldn't worry as much about rpm's specifically. More important is airflow (cfm), noise (dba) and if the fans have to pull air through a filter or push air past hard drives or anything to some degree static pressure will have an impact. I'm not sure if you're looking for 3pin fans or 4pin pwm fans.
Some decent 140mm fans at around $15 or less would be the fractal design gp14
http://pcpartpicker.com/product/cTH48d/fractal-design-case-fan-fdfandyngp14wt
Cougar vortex (kind of an ugly orange but maybe you're into orange or don't care)
http://pcpartpicker.com/product/j6JwrH/cougar-case-fan-cfv14h
Rosewill hyperborea
http://pcpartpicker.com/product/gD9KHx/rosewill-case-fan-rocf11003
For about $16 there's the noctua redux, they come in two tone light/dk gray vs the earthtones and use the older sso bearing rather than the sso2 but they still have a 6yr warranty. They don't come with any voltage reducers or low noise adapters, just the fan and 4 screws but they're solid quality fans and usually a bit less than the standard noctua fans.
http://pcpartpicker.com/product/8r9KHx/noctua-case-fan-nfp14sredux1500pwm
Hopefully something in there will be within your budget, they all offer decent airflow with minimal noise.