Delicate modification of the silica contents and porous structures within RHO zeolites has been readily realized by adopting a variety of alkali metal-crown ether (AMCE) complexes as the templates. Compared to the previous protocols, up to 70% of the Cs+ cations could be substituted by much cheaper K+ cations and thus the synthesis costs of RHO zeolites could be impressively reduced. The subsequent Cs-133 and Na-23 MAS NMR spectra further reveal that the Cs+ cations may aggregate with crown ether in a form of monomer or dimer complex, which then plays a significant role in the structural direction of RHO zeolites, whereas the hydrated Na+ cations mainly serve as the charge balancing cations. Meanwhile, the addition of different amount of K+ cation could result in varying degrees of template-framework interaction and consequently generates RHO zeolites with diverse compositions. Finally, the proton-type RHO zeolites were applied to the adsorptive separation of CO2/CH4/N-2 mixture. Therein, the medium-silica ones achieve fine trade-off for the adsorption capacity, selectivity and heat even under ultralow CO2 concentration, which makes them potential candidates for trace CO2 capture.