I'd wager that the most expensive is one of those extremely limited distribution ones like Engine Skydor. Beyond that, I suppose we always have the go-to answer of any MISB robo from before 1990 or so.

MISB battle fever combo 2?

After 4 years of collecting these sentai mechs, I have to say that the Bio Robo big box set is the most expensive item for sentai mech toy. But excluding box set and only look at individual item, the most expensive and hardest to find in mint/near mint condition would be bigscale denzitiger. Battle Fever battle combination 2 box set is also very rare and expensive, probably in the top ten.