Sadly, if there is no cash prize, it won't motivate the "ooRP'ers" to start RP'ing. Thing is, if we entice the ooRP'ers with cash to start RP'ing, then since they will start RP'ing more, they will get used to it. Then, they'll just Role Play because they either find it to be fun/rewarding in some sort of way.
Similar thing happened to me. When I first joined the server, I thought "RP'ing is stupid, I play this server cause lot of people here." Then, the more and more people around me Role Played, the more and more the very idea of myself RP'ing grew on me, thus bringing me to where I am today. I no longer use "lol" and smilies in any online games anymore; I almost never even use "haha" because it's too "generic."