Patricia ~ to use a scale for pre & post feed weights (test weighing,) it should be accurate to within 0.1 ounce or about 2 grams.  Most of the less expensive scales (under $300) are only accurate to 0.5 oz or 14 grams, which can be a HUGE difference when doing test weights.  This is a nice article about what factors impact test weights: 
http://www.barbarahaase.com/wp-content/uploads/2014/12/TEST-WEIGHT-ARTICLE.pdf 
Now, that said, I do test weights in my PP using a high end scale, but always caution the mom that this is just a "snapshot of a moment in time."  However, it often confirms what both the mother & I already know, that baby is not transferring adequately and I develop a plan of care accordingly.  If you want a less expensive scale for routine weights, with frequent & timely follow up, go for it.   Good luck!

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