How much does the yield of a stock, its EPS and book value factor into
the price of a stock? What are the numbers below which a stock is
considered cheap?

Hi Be11,
Stock prices are defined by the market: what an seller and buyer agree
is a fair price.  This is based on future expectations of the way the
stock price will move, which will include considerations about its
yield  - earnings per share - and its book value, but more important
will be the expectations for the future profitability of the company. 
Example:  a pharmaceutical company that has not be very profitable in
recent years but suddenly can announce that it has approval to market
a medication that is 100% effective against HIV.  Past yield and book
value  would then be of minimal consideration.
"Cheap" is relative.  The "price-earnings" relationship (share
price/dividend per share) is used to compare stock prices within an
industry.  This can/will result in some stocks' being considered
"cheap", but that is disregarding knowledgeable of future
developments.