Look on Groupon for special offers.
For a regular tube on a weekday it's 25 and weekends are 30. Same price for 2 3 and 4 hrs. If you have a group of more than 20 people then it's $19 for weekdays and $24 for weekends. However you can save probably $10 by using groupon, they aways have coupons. Also note, the prices can change frequently.
Go on their web site for information
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