Why would you require a customer cite you in code they paid for?
Stated in your OP is that customers will already receive the source code but only full rights upon citation. What are the limited rights, if any, should a customer not to cite you as the author? Citation is generally required with Creative Commons Licensing and similar licenses, not for something you directly fund.
Given your pricing model, of 1.5 to 4 BTC (currently ~$14 to $52 USD) per 750 line of code, how is this cost arrived at? What's the going rate for a if/else statements, do/while loop, or abstract base class?
But in all seriousness you should probably define the rights your customer receives. Furthermore move to a more mainstream billing model (say hourly or per project) as it will help protect both you and your customer from misunderstands/disagreements. You wouldn't want a customer saying one code segment is 1.5BTC while you think its 4BTC...