I am sort of a collector(but also game), have over a dozen rifles, I can tell you that both collecting and gaming can be equally as expensive.
I second the 4.5mm option for target shooting/plinking. I own pellet rifles/pistols and they are much more accurate and funner to shoot pop cans/targets with than airsoft guns. They can be just as expensive as airsoft guns though trust me
I'm not going to name any as you arent AV'd, but an upgraded bb gun can go for as much as $900 haha.
For what you are thinking of doing, I would say start with whatever you want, if its informal, both airsoft and pellet/bb are fun, depending on your intentions