What exactly did you see on pin 21? Was it valid looking 15kHz at 5Vp-p?
Pin 25 (enable) will only strobe low when the CIA is being read/written by the system. With DiagROM this will probably only happen during the CIA tests. I'd be a little surprised if that wasn't working, but if it's not, that's likely the problem. It's essentially an address decoded ouput from U150 (Gary) pin 46.
After that, it's just a matter of checking that you have valid clock, read/write, address bus and data bus at U300, which should all have activity present all the time regardless if the CIAs are being accessed or not.
Price wise to repair this kind of fault is about US$35 + parts + return freight, it's probably not complicated. Capacitor replacements at the same time will be approx + US$40; includes full testing. Sounds as though there is probably battery corrosion damage to clean up as well, price varies depending on the extent of the corrosion damage.
Contact me for more detail if needed:
http://amiga.serveftp.net
