Safe Rooms that can withstand an F5 can net-cost as little as $500: http://tinyurl.com/pq346hf

A safe room, built to FEMA specs and designed to withstand 250MPH winds,
will cost only around $2,500; and FEMA (and/or other programs) may pick
up the first $2,000 or so.

http://www.slate.com/articles/health_and_science/explainer/2013/05/oklahoma_tornado_storm_shelters_where_is_the_safest_place_in_a_tornado.html

http://www.fema.gov/library/viewRecord.do?id=1536