As for cheap CO2 manifolds, look for air tool couplers/manifolds/splitters. Harbor freight used to carry a 4 port manifold for $1.99, but that appears to be discontinued (still available on
amazon but for nearly $4). They also have
this manifold for $10, which comes with some quick connect air fittings that can be removed and replaced with 1/4" flare check valves (about $2.50 each at NB), or shutoff valves with intergrated check valves if you want to get spendy (which may drive the price back up to the $40 range).
Another good alternative are air manifolds for
aquariums, which are nothing more than a tube with a bunch of valves in it. For these you'd need inline check valves to prevent gas mixing or liquid backup. You could also just buy a large piece of copper or steel pipe and a set of pipe taps from
harbor freight for $14 if you wanted to build your own from scratch.