BER advise

If a BER reports that:
1.insulating walls have a medium cost with low energy savings
2.replacing windows have a medium cost with medium savings
am I right in assuming that spending money on option 2 is the best thing to do?

hmm...the BER is a very blunt instrument. You need to weigh it up for yourself.

They are also just template answers which are not suitable in the real world. Depending on your house, it could cost upto €30k for replacement windows but on average €15k. I wouldn't considered that medium cost. The benefit of replacement windows would be dependent on what they are replacing, the size & orientation of the windows, size compared to wall area, existing wall insulation levels, U value of the replacement windows, etc.
Insulation would be on average €2k, which it would consider medium cost.
Things like sealing draughts, etc. IMO would be low cost.