How to ask for a research project fee to an external investigator?
Pick up the conversation from that 'lead' s/he gave you and work forward from there:


  Hi [Name],
  
  When we last talked you mentioned the funding situation. You said there isn't currently any funding specific to the project. So - can you let me know what funds you guys right now as you work on it?


So you would already be insinuating that people working on the project need to be funded, without explicitly saying INSERT COIN. Then depending on his/her answer I would suggest that they/you all apply for that project-specific funding, without actually agreeing to start work - implicitly assuming (or proposing the assumption, if you will), and thus insinuating, that such funding is a prerequisite for you to start working. 

If the message doesn't come across increase the strength of the hinting while giving some (but not too much) way preparing for the actual collaboration.