You can go on Xbox Live with a Silver Account which is free. That only entitles you to purchase stuff from the marketplace though. If you want online gaming you'll need an Xbox Live Gold Account which ranges in price depending on if you want a 1 month, 3 month or 12 month subscription.

UK prices are:

1 month - £9.99
3 month - £19.99
12 month - £39.99

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