A 24hour plumber will charge more especially when he has to come to your house at 2am. He charges for fast service, and since other plumbers are not willing to show up after hours, the 24hour plumber can charge what ever he wants. Their rate also depends on the distance they have to travel from their shop. If he lives an hour away from you that is already $65-100 drive just to your house. Granted most decent plumbers will only charge you for that drive time if they do the work. So if it takes him half hour to get to your house and he does 30 min of work he will charge you his rate 1h + the parts.

If word-of-mouth doesn't find you someone good, call the guys in the phonebook/paper with individual names, not companies. No/lower overhead, and usually they have more experience than the people the companies are turning loose. Roto-rooter doesn't have "real" plumbers for all jobs. They have kids ride along with another "rooter" for a few weeks, then the company turns them loose.
You'll almost always get better prices, and better service from "Joe Schmoe, Master Plumber" listings.
(advice MY plumber gave me last week. When he came out from 8-11pm to use the "rooter" thing to remove a years worth of root bundles clogging my line to the sewer... and charged me $120 cash, and a bottle of Iced Tea.)

I gotta second @stupimlico on the solder vs. weld (for copper pipes) and using sharkbite, as well. I installed a water heater recently with sharkbite fittings on PEX pipes, but it works on copper and PVC as well. Be sure you find the house cutoff valve before you start cutting! ;-)

Might be worth signing up for Angie's List if available in your area.

Also, if you have time, patience, and are willing to learn (and make mistakes) as you go, a lot of small plumbing jobs can be DIY and save you big $$$. Google the problem, pick up a basic how-to book, then go to your local plumbing supply place for tools, supplies and much better advice than you'll get at the big box store. New shut-offs are fairly easy to install: shut off water, cut pipe, smooth/clean pipe with emery paper, clamp on compression fitting with wrench, test for leaks, done.

ETA: OK I was slow, others have already covered the same ground.

@almirar: I would think a handy man would be much cheaper. Most other plumbers would be much cheaper.

Might they have said it needed to be soldered, instead of welded? If you did need a new shut off valve soldered on then there is a good chance your pipes are copper and if they are copper then there should be a good selection of push fittings at a lowes, home depot or plumbing supply place somewhere near you. These fittings only require you to cut the pipe squarely, clean the pipe of solder or dirt and push the fitting/valve onto the pipe. The fittings are a little bit on the expensive side(as far as fittings go) but they would only be a fraction of the cost of a plumber at the rate you were quoted. Here's a link, this brand is sold at the Lowe's where I live. http://www.sharkbite.com/usa/

Obviously, I would only suggest something like this for someone who felt comfortable doing so.

Best of luck to you.

@w00tgurl: I usually do my own, but the installers said it needed to be welded on, and that's beyond my ability... but he did suggest a handiman instead of a plumber, is that cheaper?

that's why i do my own, unless it's stuff i can't control since i live in a building. pick up a home improvement 101 book from home depot, some supplies like pipes, fittings, etc and try some preventative maintenance
or if you just don't want to, once you find a plumber you trust from references, get some of those issues taken care of all at once before the premium rates on the weekends and at night. and before you absolutely have to get stuff done, because by then beggars can't be choosers. especially check every shutoff valve, sprinkle some baby powder and stick a flashlight around it to see if you have leakage
and get some leakfrogs or a similar leak warning gadget, to warn you of leaks earlier

I had a plumber come out on New Year's Eve day to change a zone valve motor on my boiler. Minimum one-hour labor charge (on overtime) was $89.25 plus parts. He was here around a half hour.
And damn, Honeywell, you are sure proud of your repair parts!

The best thing you can do is what @moosezilla suggested... ask everyone you know if they can recommend a good, yet cheap, plumber. I found one that way. Very reasonable. Word of mouth really does work. The same people will tell you who NOT to call, too. Some of us learned the hard way. ;-)

I had to have a blocked kitchen drain worked on three weeks ago. The company sent two plumbers, one of whom had to clamber up onto the roof to snake a pipe. They double checked our bathroom to make sure there was no other blockage (it's an old house with old plumbing), recommended we consider having our septic tank pumped out (not a service they provide), and were here almost exactly an hour.

Final bill: $98.00.

AAAAahhhhh, the joy of unions.

Every plumber that I've encountered for the past 5 years has used what they claim to be a standardized book of rates for specific jobs, rather than hourly. Those rates always seemed a bit high, but comparing them to your quote, it's not bad. Get more quotes.