How much labor cost should I expect to pay to have them changed out? Also, what is a good cost on the new springs?

I believe this is correct for the 500's:

Comp 292
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Comp 292 (16) $124.99 929-16 - Comp Cams Dual Valve Springs -- Jegs and Summit Racing has them

Isky 8205 Plus
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http://www.iskycams.com/pdfcatalog/2004-05/page42.pdf
Isky Racing 8205 Plus (16) $285.95

Is it true that only the early "carb" 500hp's had the valve spring problems ? According to previous threads, the HP500's run camshafts with high lift and steep opening and closing rates which was hard on valvetrain components. Does anybody know what years the 500's have this issue or is it for all of them ? Why doesn't the 500 EFI have the same issue or does it ?

The early HP500's had problems with reversion which led to a lot of moisture in the engine and rusting valvesprings. Flawed valvesprings break easier. Mercruiser responded by changing the exhaust to eliminate the reversion and they changed the valvesprings to a more durable version. I think the problem was solved by 1998 but the valvetrain is still hard on parts and needs more maintanence than the 496's.

Don't forget to change the lifters when you do a valve job or change the springs. Roller lifters on high lift cams fail frequently and when they do the results are disasterous. Pieces of metal go everywhere inside the engine. It gets embeded in the piston skirts and ruins the bores. Don't assume the lifters are good for 1000 hrs. Wonder how I know this?

GET IT DONE. 300 hours is way to many and pushing your luck. One second running strong, next second, bilge filled with oil and major damage. Not putting a damper on things, but it has been talked about 200+ times on OSO. Just do a search. Don't take the chance. It is cheap to do. It will be ridiculously expensive if you don't. Just 2 cents

How much should I expect to pay for labor on this job? My reason for asking, is I just got a huge bill just for impellers and drive servicing, and I want to make sure that I don't have any surprises.

Labor shouldn't be that much. The springs can be replaced without pulling the heads by putting compressed air in the cyld and using a screw type spring compressor. If you have any mechanical skill you can do it yourself in a couple hours. Now, the question is, where do you get the springs? Crane Cams is history.

i paid $1200 for twins including all parts (and plugs) but that was over a year ago.

jim