Multiple factors x ($ I purchase) = 1.00 ($ / purchase) Cost of purchase of'/ lb is $1.00.

Let x equal what you are trying to find
x = ($ purchase) Note: The unit "purchase" is made up.
Reciprocal factors
16 (oz / lb) 1 / 16 (lb / oz)
4 ($ / lb) 1/4 (lb / $)
Build an equation
x ($ / purchase) = 4.00 ($ / lb) * 1 / 16 (lb / ez) * 10 (e-z / purchase)

Multiply factors
x ($ / purchase) = 2.50 ($ / purchase) Cost of purchase of 10 oz is $2.50.