You need the EFL license for VR and VRRP, and in a VC you need one for each switch.
Virtual Chassis doesnt require a license in any EX series.
You can't buy a license thata only enables the feature you need at a set price, it doesnt work that way with Junos licenses.
If your EX2300-C for some odd reason requires a license, the EFL license will cost you more, than just upgrading the hardware to e.g. EX3300-24 switches.