The main reason a mobility card would be used in an all in one machine such as an iMac is because the mobility cards are smaller, lower power, and cooler. You sacrifice performance because of it though.
Of course, if you aren't gaming or anything, then a mobility 5850 is more than adequate.
Oh, and also, unless you really like the design of the iMac, you can do a hell of a lot better for the price.
What I like is the beautiful panel, and the fact that Apple is able to pack a computer into a monitor by only adding an inch and a half of bulge.
And yes of course you can get more for your money, I'm running an i7, p6t and 4870 for nearly half the cost.
But my complaints are:
dust - I replace/clean my case filters nearly every month
heat - warms the room 5-10 degrees
power - running at 450watts my monthly bill fluctuates nearly $30
Apple tends to optimize their products and make huge profits based on older but highly efficient hardware/software. As far as gaming, you most likely wouldn't even be able to play anything on the native resolution with full specs, which is ludicrous.
I mainly wanted to know the cost of the 5850 mobility card, would it make sense to get it over the 5850.
Supposedly the performance of the 5850 mobility is on par of the discrete 4850.