Short answer: Park at Long Rock (cheap), walk the bay, cross the causeway for free, do the rock pools, marsh and museum, eat pasties on the wall and finish with a drink at The Fire Engine. Comfortably under £50 for four.
St Michael's Mount tickets for a family of four can run to the wrong side of £60 before you've bought a boat or a coffee. You can have a genuinely good Marazion day without going through the gate. Here's one, with the numbers.
Park at the station car park by Jordan's Cafe at Long Rock rather than the long stay in town — it's a fraction of the price and on the beach. Walk east along the sand or coast path with the Mount in front of you, 20 minutes. Parking: about £4.
Time it for the causeway opening and walk across to the island harbour. Crossing is free; you just can't go through onto the island itself without a ticket in season. Climb Chapel Rock for the photo on the way. Rock pools either side of the causeway and around the harbour at low tide will absorb children for as long as you let them. £0.
Pasties from Philps Bakery eaten on the sea wall, and an ice cream each from Ruby's Ice Cream down the alley. Roughly £30 for four.
Ten minutes' walk west to RSPB Marazion Marsh — flat, free, herons and egrets, a good half-hour. Then back through town to Marazion Museum, free, twenty minutes, with the old gaol out the back (closed Wednesdays and weekends). £0.
The Fire Engine up the hill has picnic benches that look straight down the bay to the castle. Two pints and two soft drinks. About £14.
Total: around £48, and you've seen the Mount from closer than most people who paid.
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