This actually depends on the case. Some cases it's just 4 screws. Others you have to dismantle the entire case.
The main things you want to look at is make sure the new one is the same type (ATX, microATX, ATX2.0, etc). Voltages usually don't matter because normally there is a switch on the back that you can change (120V or 230V) you want to look at wattage. Do not buy one with a lower wattage rating than what you have now. Pay attention to the rails (3.3v, 5v, 12v) make sure the amperage rating is the same or higher than your present PSU. Also make sure you have the same connectors as the one being replaced.
Price of PSU depend on the quality and the output power you want. they can be as little as $12.00 (for cheapo budget unreliable models) or as high as $200.00. Average 'good' PSUs usually run around $50.00.
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