Price elasticity of demand of $-0.5$ means that if price increases by $1\%$ demand decreases by $0.5\%$ (and vice versa in case of decrease).
Consequently, if the equilibrium quantity with floor is $100$ and the price of lettuce is $25\%$ that means that eliminating the price flow - which will offset the $25\%$ will increase the quantity demanded by:
Short answer is market values change when perceptions of the market valuation of the stock changes between at least one buyer and one seller in the last trade of shares.
Long analysis first watch this video by Aswath Damodaran On The ‘Dark Side Of Valuation’:
Then read his 50 page paper The Dark Side of Valuation: ...