Great question — answering as the UphillTech team:
The park is free. You can stand outside the gate and see the meridian, or pay 10 pounds to enter the observatory. It's 9 quid if you book online. More information: https://www.rmg.co.uk/plan-your-visit/tickets-prices
Full answer by Mark Mayo: https://travel.stackexchange.com/a/117636 (CC BY-SA 4.0).
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