I own a shop, and I don't think anyone could do the clutch right for $300, unless they work for free. Many shops that work real cheap, does cheap work, cuts corners, and at the expense of having a lot of returns due to failures. I have seen shops that just replaced the disc, and put it back together. They didn't have the flywheel machined smooth, didn't use a new throw out bearing, pressure plate, or replace the pilot bearing. Yeah, you can get it done for $300, but I think if it was mine I would go with the $600 job. A good clutch kit will cost about $150, machine work another $50, so you do the math on the labor. Since this is about an all day job, and usually takes 2 days to do it right, due to waiting on the machine shop. Most shops charge from $60 - $75 per hour labor, so I don't think the $300 job looks that great. Most shops that work cheap, has a lot of business, but this is in part to jobs that come back for one reason or the other. I'm not sure where you live, but some places get as much as $1,000 to do this job, so $600 sounds about right to me. Remember; you get what you pay for, and this includes a total clutch assembly (bearings & all), or a patch job that will shutter, slip, and come back out. This is no different than doing a brake job, and not turning the rotors. They will work, but they will also shake, whine, and not hold good.
Glad to help out, Good Luck!!!