Are you planning on allowing any variety of quantity choices? In otherwords, do you really want your customers typing in 5 and a half pounds or 5.5 lbs? Using the text entry prevents the system from calculating your totals and surely slows down your processing of orders and allows for some ambiguity about what the customer really wants.
Why not restrict them to specific quantities? For instance, if granny apples can have some standard packs (by weight) you could make the option values to match.
Example:
Option Name Option values
pound(s) 1
pound(s) 2
pound(s) 3
pound(s) 4
pound(s) 5
pound(s) 10
pound(s) 20
pound(s) 30
pound(s) 50
pound(s) 100
That way, if the customer wants 5lbs he selects 5 pound(s) and then enters a quantity of 1 (unit) in the add to cart box signifying that he wants 1 x 5lb pack. Likewise, if he wanted 200 lbs he would select the 100 pound(s) units and enter 2 (units) in the add to cart box to signify he wants 2x 100lb packs.
It takes a while to enter all the different option values, but it is only done once and relatively easy to add new ones.
The option names and associated option values only need to be entered once, but can be used by all products needing that option. In otherwords, if you sell apples, oranges and pears by the pound, then the attributes tables for those products can reuse the option name/option values previously set up for pounds. And you can pick and choose which option values to use for each product. If you only sell oranges in 10 and 50 pound sacks, then you can choose to only display 10 and 50 pouind option values on the product page for oranges.