what or how much to give as an engagement gift to a coulple where the
girl is of Indian origin?

This Indian Wedding Site has experts who answer such questions: 
http://www.shaadi.com/wedding/planning/experts/ask-our-experts.php?expert_type=planning

Part 2 answer is give what you feel is appropriate, and within your
means.  If Indian tradition calls for more than you can afford, don't
go into debt to give a gift.

Since you don't mention what your budget would be, be creative.  Shop
at antique stores or second hand stores for a very unique piece of
glassware, or china, silver etc that you can afford.  Or give the
bride something personal like a massage, pedicure and manicure etc. at
your favourite salon.  Home made items are very personal if you paint,
sew, craft or otherwise and are unique and treasured by people who
know you.  No gift is too small when given from the heart.  I perform
many weddings every year, and most people just want to know that you
share their happiness and excitement.  Read my testimonials and you
will see what I mean.  http://www.mts.net/~quin52