[AccessD] New engagement; trying to avoid the "tip of the iceberg" consulting trap

I'm going in to see a new client (via agency) on Monday.
The deal is supposedly this:
I interview with them for an hour; assess the project scope and duration.
If they like what they hear, then I'm hired.

Anyone seasoned in consulting can immediately see the "trap" that's being
set.
I want to avoid it by only giving them a rough estimate AFTER spending 8
hours with the current code base.
I've seen way too much fuggly VBA code to be duped into giving a blind
estimate.
I'm also afraid they'll only be showing me the "tip of the iceberg".

Only question remains: if my estimate at the end of the day is rejected,
what do I charge for the 8 hours that they will consider wasted ? Half rate
? Full rate ?
In my mind, this is like an initial legal consult...and lawyers usually give
you 1 hour for half rate.