As mentioned, the economy in EVE is solid, and pretty much everything revolves around it: you use it to purchase ships, modules, blueprints, skills, materials, stations and corporation management, and you can incur significant losses when your best ship blows up or your corporation loses everything in a war. The cost of a monthly license is about 3-400 million ISK, which is fairly substantial and requires having already invested some time in developing a character. I think CCP struck the right balance.
With upcoming economical changes, we'll have to see if Ryzom permits this kind of addition, but under the previous (existing) model, it would not have worked as Dappers have no sink (Mektoubs and teleports, that's about it); the PLEX equivalent would probably have had to be around 40-50M Dappers or so, which only master crafters could possibly afford by grinding or going for boosts...
With upcoming economical changes, we'll have to see if Ryzom permits this kind of addition, but under the previous (existing) model, it would not have worked as Dappers have no sink (Mektoubs and teleports, that's about it); the PLEX equivalent would probably have had to be around 40-50M Dappers or so, which only master crafters could possibly afford by grinding or going for boosts...