Normally I would say you are always better off feeding them yourself, but the $50 does change things a little bit, but not as much as you would think.  We have our milk replacer for the coming year, but not the price of it yet(long story), but last year we paid $28 a bag, and it takes less then a bag to wean a lamb.  At 25 lbs the wife yanks them. 

Figuring $50 for the lamb, and $25 for milk, that's $75, and once finished, we got $267 each for a fat lamb last summer.

Another way to look at it, you should be getting an average of 2 lambs per ewe, so you are getting only $100 after buying and running after the ewe.

It's a tough call.  I know we would still elect to finish ours rather then sell, even if offered $50, but that's probably just us.