If we're talking about a truly VFR-only plane (i.e., one with nothing more than the 91.205(b)-required Day VFR equipment), and you want a full, 21st century IFR panel (i.e., all required instruments, dual nav/comm, transponder, audio panel/intercom, IFR WAAS GPS,2-axis autopilot, etc), you're probably talking about writing a check for at least $50K to your avionics shop. OTOH, if you're talking about taking a plane with the "usual" VFR package seen in most primary trainers (6-pack instruments, single nav/comm, transponder with encoder, audio panel/intercom) and bringing it to what I'd consider a comfortable IFR package with simple autopilot (like an S-Tec 20), that could be done for under $20K, or less if you don't put in a GPS.