How to ask for a research project fee to an external investigator? I'm currently a postdoc, and someone from another institution (a colleague of a colleague; I didn't know them before) contacted me to invite me to collaborate on their project. 

While the project seems interesting, I would need to invest my own time outside my normal working hours, so I'm not willing to take it without proper compensation.  

Basically, I would like to ask for a fee. The person who contacted me said something a bit ambiguous "we have no money, but we could probably get some". Do you have any advice on how to politely ask for a fee?
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.