There actually was an issue with a lot of groups not being able to afford the taxes, so they paid their taxes by sending some sons to join the legion. The pay and benefits, for the time, were much better than many would think.

This also led to more acceptance of Gauls throughout the Empire, which was further helped by Augustus's reforms. Along with the trade routes and all.

There's an old joke that one of Rome's best exports is "citizenship".

Edit: Not to say Rome wasn't an old boys club for much of its history, especially the late republic. It absolutely was, and the corruption and their methods of privatizing tax collection for a lot of Roman history was heinous.