Sounds a tad high to me. Its been a few years since I paid a mechanic to replace a Volvo timing belt. Last time was in 1996 on my 240 (essentially the same amount of work and same parts as a 740). I paid $30 for parts (including new crank & camshaft seals) and $70 for labor (less than 2 hrs). So with inflation, you might expect to pay $150 or so now?. Maybe someone else has more recent data.
Its fairly straightforward to do it yourself and there is plenty of info in FAQ and archives - its one of those jobs that gets questions posted very frequently. You may need to buy a crankshaft pulley "counterhold tool" to hold the pulley while you loosen the bolt. This is $75 or so for the Volvo OEM tool or $45 or so from IPD for their equivalent aftermarket tool. There are other ways of imobilising the pulley (see FAQ) but using the tool makes the job easier. You will also need a BIG breaker bar to loosen the pulley bolt. An 18" bar with a 4ft pipe slid over the handle usually does the trick. The accepted wisdom is that you should replace the camshaft, intermediate shaft & crankshaft seals while you are doing it. If its the first time you are doing a timing belt you should set aside a weekend. you may get done sooner, but if not you will have time to deal with unexpected snags along the way.
Good luck.
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Current: '95 945, '90 BMW 325i. Former: '85 245, '91 744T