Perhaps you could (also) add a credits subscription which will give a standard amount of credits every month at the best available price (beats any package price). For $9.99, give 250 at first month (standard rate) and 325 credits for every month after that. This way you can build a base and build on it. I know I would take this deal not necessarily for the need of the credits but mostly because I want to see this game progress.
For more free credits options, you could throw guilds into the mix.
-Give 25% (or at least 10%) of the credits a player gets in his first purchase at the guild leader who first got him in a guild.
Let the leader decide whether or not he needs to split this sum up; he knows better than all who got mostly involved in the particular player's progress. Every guild could also use its own rules on how this splitting will take place. (for example, recruiter gets 10%, teachers gets 80% and the leader keeps 10%). In this way, you will give extra bonuses to guilds to not only train newcomers fast but also "push" them into credits. It is a win-win situation for all.