Guys, guys, guys I know all about what you're holding my hand through. However, I know the common conception if that the Centurions were "promoted upwards", and there were no barriers (technically) to that happening. There's just such a lack of any solid evidence of such things happening(and Centurions were fairly well documented, since their commanders treasured them so well) and on closer inspection the overwhelming majority of Centurions were middle to upper middle class people, for the reasons I've already mentioned, and for the reasons you boys have mentioned(aka any post in Roman Republic required relatively vast sums of money to acquire).