I am looking for a typical percentage of external(inter-domain)
routes versus typical percentage of internal (intra-domain) routes in
a core router with couple of hundred thousand entries in the routing
table. Can anyone please help me in this?

I think first you have to decide what a typical AS looks like. The
question, as it stands, is too general for any answer to be (in)defensible.

Assuming you are talking about service providers and not their
enterprise customers, you can probably get a fairly decent number by
looking at routing announcements for whatever you'd call a typical
network. Say...

AS (vendor L) routes / total routes or AS (vendor V) / total routes.

It's important to keep in mind that internal deaggregation for
different networks can be very high. /30s may be carried for private
interconnects as an example.