Bilbao Airport

Hello,

I am a first time visitor on this group, but I was wondering if I could
get help for my questions. I am travelling stand-by through
airtech.com to Europe from the United States in a week. I was
wondering what were the cheapest airline options to Bilbao from the
following points of origin:

London
Frankfurt
Paris

I would have to purchase my ticket on the spot (since standby is never
guaranteed and I have no fixed US departure time. So I also would need
to know which flights are less likely to be filled up and which
airlines offer on the spot purchasing for a relatively cheap price...I
would only be going one way. Basically I am trying to narrow down
which of these three cities to fly into and which airline (that is
available in Bilbao) to purchase my ticket from when I reach the
counter in one of these three cities.

I would really appreciate any advice!

Victoria