It is possible to be an FI in EASA land without a CPL. But...
You still need to have demonstrated CPL level knowledge (i.e passed the CPL ground exams!)
You need more hours/experience to be an FI without than you do with.
You can't be paid.
Don't always look at the bottom line cost wise!
The Pre-Req's and priv's for an EASA FI are detailed in Subpart J of EASA PART FCL (Google 1178/2011R)