Ok. So you need to move up your timetable. Contact your customers, explain the situation. If the service is as valuable to them as you say it is then they will be happy to pay. $25000 over 18 months works out to a shade under $1400/month. Divided by 300 active users works out to less than $5 / month for each. Call it $10 to build in some safety. Let them know that not charging for the product any longer is no longer an option, but that because they are the first 300 they will get a 40% discount on what everybody else will be paying from now on: $25.

If what you say is true then your problems will be solved, if you are making an error of assumption then it will surface and maybe you will be able to address it, or maybe not and then you will have the answer to your question.

Best of luck!