We're in North Yorkshire, within around 45 minutes drive from the coast. We have always avoided the seaside resort type places, like Scarborough - not only because of the money spending potential, but also because as soon as my DCs cast their beady eyes towards the funfair rides, arcades etc, they become far less content with making sandcastles, exploring rock pools etc.
We tend to head to Hunmanby Gap. Miles long stretches of beautiful sand, shallow water, and only a beach hut cafe and toilets.
We always take a picnic, which is just general stuff from home - sandwiches, scotch eggs, crisps, fruit, cold drinks sort of thing. Other times, we'll bring a disposable bbq, with a packet of sausages and some rolls (although that adds hassle of needing to pack tomato ketchup/bbq sauce/plastic cheese slices/utensils, etc).
We pack buckets and spades, cricket set, frisbee, football, and the DCs can spend an entire day on the beach.
Sometimes, if the weather turns, we'll head into Filey and go to the arcades at the end of the day. We'll put £4 into the change machine to get 2p's - once that's done, we're done.
We'd normally finish the day with an ice cream, so that would maybe be £4/5 for the two DCs, DH and I don't generally have one.
So, a day out at the beach for us would be the cost of fuel to get there, parking £5 for the whole day (other beaches locally are free), cost of picnic (but we'd be eating lunch at home anyway), ice cream and (sometimes) arcades £10. Maybe £30 in total.